Stein Vidar Hagfors Haugan

Publications from ADS

• Aussel, H.; Tereno, I.; Schirmer, M.; Alguero, G.; Altieri, B. & Balbinot, E. [Show all 409 contributors for this article] (2026). Euclid Quick Data Release (Q1): I. Data release overview. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554610. Full text in Research Archive Show summary

  The first Euclid Quick Data Release, Q1, comprises 63.1 deg2 of the Euclid Deep Fields (EDFs) to the nominal wide-survey depth. It encompasses visible and near-infrared space-based imaging and spectroscopic data, ground-based photometry in the u, g, r, i, and z bands, as well as corresponding masks. Overall, Q1 contains about 30 million objects in three areas near the ecliptic poles around EDF-North and EDF-South, as well as the EDF-Fornax field in the constellation of the same name. The purpose of this data release – and its associated technical papers – is twofold. First, it is meant to inform the community of the enormous potential of the Euclid survey data, to describe what is contained in these data, and to help prepare expectations for the forthcoming first major data release DR1. Second, it enables a wide range of initial scientific projects with wide-survey Euclid data, ranging from the early Universe to the Solar System. The Q1 data were processed with early versions of the processing pipelines, which already demonstrate a good performance, with numerous improvements in implementation compared to pre-launch development. In this paper, we describe the sky areas released in Q1, the observations, a top-level view of the data processing of Euclid and associated external data, the Q1 photometric masks, and how to access the data. We also give an overview of initial scientific results obtained using the Q1 dataset by Euclid Consortium scientists, and conclude with important caveats when using the data. As a complementary product, Q1 also contains observations of a star-forming area in Lynd’s Dark Nebula 1641 in the Orion A Cloud, observed for technical purposes during Euclid’s performance-verification phase. This is a unique target, of a type not commonly found in Euclid’s nominal sky survey.

• Bergamini, P.; Meneghetti, M.; Acebron, A.; Clément, B.; Bolzonella, M. & Grillo, C. [Show all 378 contributors for this article] (2026). Euclid Quick Data Release (Q1): XXXIII. The first catalogue of strong-lensing galaxy clusters. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554577. Full text in Research Archive
• Hill, R.; Abghari, A.; Scott, D.; Bethermin, M.; Chapman, S. C. & Clements, D. L. [Show all 304 contributors for this article] (2026). Euclid Quick Data Release (Q1): XXIII. The average far-infrared properties of Euclid-selected star-forming galaxies. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202557514. Full text in Research Archive Show summary

  The first Euclid Quick Data Release (Q1) contains millions of galaxies with excellent optical and near-infrared (IR) coverage. To complement this dataset, we investigate the average far-IR properties of Euclid-selected main sequence (MS) galaxies using existing Herschel and SCUBA-2 data. We used 17.6 deg^2 (2.4 deg^2) of overlapping Herschel (SCUBA-2) data, containing 2.6 million (240 000) MS galaxies. We binned the Euclid catalogue by stellar mass and photometric redshift and performed a stacking analysis following SimStack, which accounts for galaxy clustering and bin-to-bin correlations. We detected stacked far-IR flux densities across a significant fraction of the bins. We fitted modified blackbody spectral energy distributions in each bin and derived mean dust temperatures (Td), dust masses (Md), and star-formation rates (SFRs). We find similar mean SFRs compared to the Euclid catalogue, and we show that the average dust-to-stellar mass ratios decreased from z  ≃  1 to the present day. Average dust temperatures are largely independent of stellar mass and are well-described by the function T2  +  (T1  −  T2) e^−t/τ, where t is the age of the Universe, T1  =  (79.7 ± 7.4) K, T2  =  (23.2 ± 0.1) K, and τ  =  (1.6 ± 0.1) Gyr. We argue that since the dust temperatures converge to a non-zero value below z  =  1, the dust is now primarily heated by the existing cooler and older stellar population, as opposed to hot young stars in star-forming regions at higher redshifts. We show that since the dust temperatures are independent of stellar mass, the correlation between dust temperature and SFR depends on stellar mass. Lastly, we estimate the contribution of the Euclid catalogue to the cosmic IR background (CIB), finding that it accounts for > 60% of the CIB at 250, 350, and 500 μm. As the Euclid mission progresses, larger catalogues will allow us to probe the far-IR properties of MS galaxies out to higher redshifts and lower stellar masses, potentially recovering the complete CIB.

• Roster, W.; Salvato, M.; Buchner, J.; Shirley, R.; Lusso, E. & Landt, H. [Show all 318 contributors for this article] (2026). Euclid Quick Data Release (Q1): XVI. Optical and near-infrared identification and classification of point-like X-ray selected sources. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554616. Full text in Research Archive Show summary

  To better understand the role of active galactic nuclei (AGN) in galaxy evolution, it is crucial to work with a complete and pure AGN sample. X-ray surveys are key to doing so, but their larger positional uncertainties complicate counterpart (CTP) association, further compounded by the limited availability of deep, uniform multi-wavelength ancillary data. Euclid is revolutionising this identification effort, offering extensive coverage of nearly the entire extragalactic sky, particularly in the near-infrared bands, where AGN are more easily detected. Using the first Euclid Quick Data Release (Q1), we validated the methods for identifying and classifying Euclid CTPs of known point-like sources from major X-ray surveys, including XMM-Newton, Chandra, and eROSITA. Using Bayesian statistics, combined with machine learning (ML), as incorporated in the algorithm NWAY, we identified the CTPs of 11 286 X-ray sources from the three X-ray telescopes. For the large majority of 10 194 sources, the association is unique, with the remaining ∼10% of multi-CTP cases equally split between XMM-Newton and eROSITA. Six percent of the Euclid CTPs are detected in more than one X-ray survey. We then used ML to distinguish between Galactic (8%) and extragalactic (92%) sources. We computed photo-zs using deep learning for the 9259 sources detected in the tenth data release of the DESI Legacy Survey, reaching an accuracy and a fraction of outliers of roughly 5%. Based on their X-ray luminosities, all CTPs identified as extragalactic are classified as AGN, most of which appear as type I AGN according to their hardness ratios. With this paper, we release our catalogue, which includes identifiers, basic X-ray properties, the reliability of the associations, and additional property extensions, such as Galactic- or extragalactic classifications and photometric/spectroscopic redshifts. We also provide probabilities for sub-selecting the sample based on purity and completeness, in order to allow users to tailor the sample according to their specific needs.

• Duffy, C.; Cappellaro, E.; Botticella, M. T.; Hook, I. M.; Poidevin, F. & Moriya, T. J. [Show all 177 contributors for this article] (2026). Euclid: Quick Data Release (Q1) – Photometric studies of known transients. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554600. Full text in Research Archive Show summary

  We report on serendipitous Euclid observations of previously known transients, using the Euclid Q1 data release. By cross-matching with the Transient Name Server (TNS) we identify 164 transients that coincide with the data release. Although the Euclid Q1 release only includes single-epoch data, we are able to make Euclid photometric measurements at the location of 161 of these transients, where 67 had a significant flux measurement in at least one band. Euclid obtained deep photometric measurements or upper limits of these transients in the I_E, Y_E, J_E, and H_E bands at various phases of the transient light curves, including before, during, and after the observations of ground-based transient surveys. Approximately 70% of known transients reported in the six months before the Euclid observation date and with a discovery magnitude brighter than 24 were detected in Euclid I_E images. Our observations include one of the earliest near-infrared detections of a Type Ia supernova (SN 2024pvw) 15 days prior to its peak brightness, and the late-phase (435.9 days post peak) observations of the enigmatic core-collapse SN 2023aew. Euclid deep photometry provides valuable information on the nature of these transients such as their progenitor systems and power sources, with late-time observations being a uniquely powerful contribution. In addition, Euclid is able to detect the host galaxies of some transients that were previously classed as hostless. The Q1 data demonstrate the power of the Euclid data even with only single-epoch observations available, as will be the case for much larger areas of sky in the Euclid Wide Survey.

• Marca, A. La; Wang, L.; Margalef-Bentabol, B.; Gabarra, L.; Toba, Y. & Mezcua, M. [Show all 336 contributors for this article] (2026). Euclid Quick Data Release (Q1): XVII. First Euclid statistical study of galaxy mergers and their connection to active galactic nuclei. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554579. Full text in Research Archive Show summary

  Galaxy major mergers are indicated as one of the principal pathways to trigger active galactic nuclei (AGN). We present the first statistical analysis of the major merger and AGN connection in the Euclid Deep Fields, and showcase the statistical power of the Euclid data. We constructed a stellar-mass-complete (M★ > 10^9.8 M⊙) sample of galaxies from the quick data release (Euclid Quick Release Q1 2025) in the redshift range z = 0.5–2. We selected AGN using X-ray detections, optical spectroscopy, and mid-infrared (MIR) colours, and by processing I_E observations with an image decomposition algorithm. We used convolutional neural networks trained on cosmological hydrodynamic simulations to classify galaxies as mergers and non-mergers. We found a larger fraction of AGN in mergers compared to the non-merger controls for all AGN selections, with AGN excess factors ranging from two to six. The largest excess we observed was in the MIR AGN. Likewise, a generally larger merger fraction (f_merg) was seen in active galaxies than in the non-active controls, with the excess depending on the AGN selection method. Furthermore, we analysed fmerg as a function of the AGN bolometric luminosity (L_bol) and the contribution of the point-source component to the total galaxy light in the I_E-band (f_PSF) as a proxy for the relative AGN contribution fraction. We uncovered a rising f_merg, with an increasing f_PSF up to f_PSF ≃ 0.55, after which we observed a decreasing trend. In the range f_PSF = 0.3–0.7, mergers appear to be the dominant AGN fuelling mechanism. We then derived the point-source luminosity (L_PSF) and showed that f_merg monotonically increases as a function of L_PSF at z < 0.9, with f_merg ≥ 50% for LPSF ≃ 2 × 10^43 erg s−1. Similarly, at 0.9 ≤ z ≤ 2, f_merg rises as a function of L_PSF, though mergers do not dominate until L_PSF ≃ 10^45 erg s−1. For the X-ray and spectroscopically detected AGN, we derived the bolometric luminosity, L_bol, which has a positive correlation with f_merg for X-ray AGN, while there is a less pronounced trend for spectroscopically selected AGN due to the smaller sample size. At L_bol > 10^45 erg s−1, AGN mostly reside in mergers. We conclude that mergers are most strongly associated with the most powerful and dust-obscured AGN, which are typically linked to a fast-growing phase of the supermassive black hole, while other mechanisms, such as secular processes, might be the trigger of less luminous and dominant AGN.

• Bhargava, S.; Benoist, C.; Gonzalez, A. H.; Maturi, M.; Melin, J. B. & Stanford, S. A. [Show all 354 contributors for this article] (2026). Euclid Quick Data Release (Q1): XXXIV. First detections from the Euclid galaxy cluster workflow. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554937. Full text in Research Archive Show summary

  The first survey data release by the Euclid mission covers approximately 63 deg2 in the Euclid Deep Fields to the same depth as the Euclid Wide Survey. This paper showcases, for the first time, the performance of cluster finders on Euclid data and presents examples of validated clusters in the Quick Release 1 (Q1) imaging data. We identify clusters using two algorithms (AMICO and PZWav) implemented in the Euclid cluster-detection pipeline. We explore the internal consistency of detections from the two codes, and cross-match detections with known clusters from other surveys using external multi-wavelength and spectroscopic data sets. This enables assessment of the Euclid photometric redshift accuracy and also of systematics such as mis-centring between the optical cluster centre and centres based on X-ray and/or Sunyaev–Zeldovich observations. We report 426 joint PZWav and AMICO-detected clusters with high signal-to-noise ratios over the full Q1 area in the redshift range 0.2 ≤ z ≤ 1.5. The chosen redshift and signal-to-noise thresholds are motivated by the photometric quality of the early Euclid data. We provide richness estimates for each of the Euclid-detected clusters and show its correlation with various external cluster mass proxies. Due to the limited area and evolving data quality, the sample is not intended to serve as a reference for cosmological applications, but to verify and validate the cluster workflow. Out of the full sample, 77 systems are potentially new to the literature. Overall, the Q1 cluster catalogue demonstrates a successful validation of the workflow ahead of the Euclid Data Release 1, based on the consistency of internal and external properties of Euclid-detected clusters.

• Allen, N.; Oesch, P. A.; Bowler, R. A.A.; Toft, S.; Matharu, J. & Weaver, J. R. [Show all 311 contributors for this article] (2026). Euclid Quick Data Release (Q1): XXV. Hunting for luminous z > 6 galaxies in the Euclid Deep Fields – Forecasts and the first bright detections. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202557785. Full text in Research Archive Show summary

  The evolution of the rest-frame ultraviolet luminosity function (UV LF) is a powerful probe of early star formation and galaxy stellar mass build-up. At z > 6, its bright end (MUV < −21) remains poorly constrained due to small survey volumes of existing near-infrared (NIR) space-based imaging surveys. The Euclid Deep Fields (EDFs) will cover 53 deg^2 with NIR coverage down to a magnitude of 26.5 AB, providing a factor of 100 increase in area compared to previous space-based surveys. They thus offer an unprecedented opportunity to select bright z > 6 Lyman break galaxies (LBGs) and definitively constrain the bright end of the UV LF. With its NIR coverage extending to ∼2 μm, Euclid has the power to detect galaxies out to z ∼ 13. Here, we present a forecast for the number densities of z > 6 galaxies that Euclid is expected to observe in the final EDF dataset. Using synthetic photometry from spectral energy distribution (SED) templates of z = 5–15 galaxies, z = 1–4 interlopers, and Milky Way M-, L-, and T-type dwarfs, we investigate optimal selection methodologies for high-z LBGs in the EDF datasets. We find that a combination of signal-to-noise ratio (S/N) cuts with SED fitting (over optical to MIR bands) yields the highest fidelity sample, recovering more than 76% of the input synthetic z > 6 LBGs, while limiting low-z contamination to less than 10%. This contamination does not include effects from instrumental artefacts, which will impact the first Euclid data releases. Auxiliary data prove critical: optical coverage from the Hyper Suprime Camera and Vera C. Rubin Observatory will distinguish genuine Lyman breaks from contaminant features, while Spitzer Space Telescope/IRAC data are vital for recovering z > 10 sources. Based on empirical double power-law LF models, we expect more than 100 000 LBGs at z = 6–12 and more than 100 sources as far back as z > 12 in the final Euclid data release. In contrast, the steeper Schechter LF models predict no detections of z > 12 LBGs. In this work, we also present two ultra-luminous (M_UV < −23.5) candidates selected from the Q1 EDF-N dataset (Euclid Quick Data Release). If their redshifts are reliable, their magnitudes suggest a DPL UV LF model at z > 9. This highlights the power of Euclid in constraining the bright end of the UV LF in the early Universe and in identifying the most luminous sources that are valuable for further follow-up observations.

• Corcho-Caballero, P.; Ascasibar, Y.; Kleijn, G. Verdoes; Lovell, C. C.; Lucia, G. De & Cleland, C. [Show all 320 contributors for this article] (2026). Euclid Quick Data Release (Q1): XV. A probabilistic classification of quenched galaxies. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554582. Full text in Research Archive Show summary

  Investigating what drives the quenching of star formation in galaxies is key to understanding their evolution. The Euclid mission will provide rich spatial and spectral data from optical to infrared wavelengths for millions of galaxies, and enable precise measurements of their star formation histories. Using the first Euclid Quick Data Release (Q1), we developed a probabilistic classification framework that combines the average specific star-formation rate (sSFR_log(τ)) inferred over two timescales (τ = 10^8, 10^9 yr) to categorise galaxies as ‘ageing’ (secularly evolving), ‘quenched’ (recently halted star formation), or ‘retired’ (dominated by old stars). We validated this methodology using synthetic observations from the IllustrisTNG simulation. Two classification methods were employed: a probabilistic approach, which integrates posterior distributions, and a model-driven method, which optimises sample purity and completeness using IllustrisTNG. At z < 0.1 and M* ≳ 3 × 10^8 M⊙, we obtain Euclid class fractions of 68–72%, 8–17%, and 14–19% for ageing, quenched, and retired populations, respectively, which is consistent with previous studies. Ageing and retired galaxies dominate at the low- and high-mass end, respectively, while quenched galaxies surpass the retired fraction for M* ≲ 10^10 M⊙. The evolution with redshift shows increasing and decreasing fractions of ageing and retired galaxies, respectively. The fraction of quenched systems shows a weaker dependence on stellar mass and redshift, varying between 5 and 15%. We find tentative evidence that more massive galaxies usually undergo quenching episodes at earlier times with respect to their low-mass counterparts. We analysed the mass-size-metallicity relation for each population. Ageing galaxies generally exhibit disc morphologies and low metallicities. Retired galaxies show compact structures and enhanced chemical enrichment, while quenched galaxies form an intermediate population that is more compact and chemically evolved than ageing systems. Despite potential selection biases, this work demonstrates Euclid’s great potential for elucidating the physical nature of the quenching mechanisms that govern galaxy evolution.

• Laigle, C.; Gouin, C.; Sarron, F.; Quilley, L.; Pichon, C. & Kraljic, K. [Show all 345 contributors for this article] (2026). Euclid Quick Data Release (Q1): XXXVI. Galaxy shapes and alignments in the cosmic web. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554651. Full text in Research Archive Show summary

  Galaxy morphologies and shape orientations are expected to correlate with their large-scale environments, since they grow by accreting matter from the cosmic web and are subject to interactions with other galaxies. We extracted cosmic filaments in projection from the Euclid Quick Data Release 1 (covering 63.1 deg2) at 0.5 < z < 0.9 in tomographic slices of 170 comoving h−1 Mpc using photometric redshifts. Galaxy morphologies are accurately retrieved thanks to the excellent imaging resolution of the visible camera (VIS) of Euclid. The distribution of massive galaxies (M* > 1010 M⊙) in the projected cosmic web is analysed as a function of morphology measured from VIS data. Specifically, the 2D alignment of galaxy shapes with large-scale filaments is quantified as a function of Sérsic indices and masses. We find the known trend that more massive galaxies are closer to filament spines. At fixed stellar masses, morphologies correlate both with densities and distances to large-scale filaments. In addition, the large volume of this data set allows us to detect a signal indicating that there is a preferential alignment of the major axis of massive early-type galaxies along projected cosmic filaments. Overall, these results demonstrate our capabilities to carry out detailed studies of galaxy environments with Euclid, which will be extended to higher redshift and lower stellar masses with the future Euclid Deep Survey.

• Lines, N. E.P.; Collett, T. E.; Holloway, P.; Rojas, K.; Schuldt, S. & Metcalf, R. B. [Show all 304 contributors for this article] (2026). Euclid Quick Data Release (Q1): XXXII. From simulations to sky: Advancing machine-learning lens detection with real Euclid data. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202558456. Full text in Research Archive
• Lines, N. E.P.; Collett, T. E.; Walmsley, M.; Rojas, K.; Li, T. & Leuzzi, L. [Show all 353 contributors for this article] (2026). Euclid Quick Data Release (Q1): XXVIII. The Strong Lensing Discovery Engine C – Finding lenses with machine learning. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554542. Full text in Research Archive
• Zatarain, T. Matamoro; Fotopoulou, S.; Ricci, F.; Bolzonella, M.; Franca, F. La & Viitanen, A. [Show all 357 contributors for this article] (2026). Euclid Quick Data Release (Q1): XX. The active galaxies of Euclid. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554619. Full text in Research Archive Show summary

  We present three catalogues of candidate active galactic nuclei (AGN) in the Euclid Quick Release (Q1) fields. For each Euclid source, we collected multi-wavelength photometric and spectroscopic information from surveys such as the Galaxy Evolution Explorer (GALEX), Gaia, the Dark Energy Survey (DES), the Wide-field Infrared Survey Explorer (WISE), Spitzer, the Dark Energy Spectroscopic Instrument (DESI), and the Sloan Digital Sky Survey (SDSS), including spectroscopic redshifts from public compilations when available. We investigated the AGN content of the Q1 fields using multiple selection methods. Applying Euclid colours and WISE-AllWISE cuts, we identified 292 222 and 65 131 candidates, respectively. We compiled a high-purity QSO catalogue based on Gaia DR3 information, containing 1971 candidates. Using spectroscopic information from DESI, we performed broad-line and narrow-line AGN selections, yielding 4392 AGN candidates across the Q1 fields. We investigated and refined the Euclid Q1 probabilistic random forest QSO population, selecting a refined sample of 180 666 candidates. Additionally, we performed spectral energy distribution (SED) fitting on sources with available z_spec, and utilising the derived AGN fraction, we identified 7766 AGN candidates. To improve the purity of the selection, we defined two new colour criteria (JH_I_EY and I_EH_gz), finding 313 714 and 267 513 candidates, respectively, across the Q1 fields. We have found a total of 229 779 AGN candidates equivalent to an AGN surface density of 3641 deg^−2 for 18 < _IE ≤ 24.5 and a subsample of 30 422 candidates corresponding to an AGN surface density of 482 deg^−2 when limiting the depth to 18 < I_E ≤ 22. The AGN surface densities recovered are consistent with predictions based on AGN X-ray luminosity functions.

• Busillo, V.; Tortora, C.; Metcalf, R. B.; Nightingale, J. W.; Meneghetti, M. & Gentile, F. [Show all 315 contributors for this article] (2026). Euclid Quick Data Release (Q1): XXXI. LEMON – LEns MOdelling with Neural networks. Automated and fast modelling of Euclid gravitational lenses with singular isothermal ellipsoid mass profile. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554538. Full text in Research Archive Show summary

  The Euclid mission aims to survey around 14 000 deg2 of extragalactic sky, providing around 105 gravitational lens images. Modelling of gravitational lenses is fundamental to estimate the total mass of the lens galaxy, along with its dark matter content. Traditional modelling of gravitational lenses is computationally intensive and requires manual input. In this paper, we use a Bayesian neural network, LEns MOdelling with Neural networks (LEMON), to model Euclid gravitational lenses with a singular isothermal ellipsoid mass profile. Our method estimates key lens mass profile parameters, such as the Einstein radius, while also predicting the light parameters of foreground galaxies and their uncertainties. We validate LEMON’s performance on both mock Euclid datasets, real lenses observed with Hubble Space Telescope (HST) that have been degraded to match observations with the same depth of the Euclid Wide Survey, and real Euclid lenses, demonstrating the ability of LEMON to predict parameters of both simulated and real lenses. Results show promising accuracy and reliability in predicting the Einstein radius, mass and light ellipticities, effective radius, Sérsic index, lens magnitude, and unlensed source position for simulated lens galaxies. The application to real data, including the latest Quick Release 1 strong lens candidates, provides encouraging results in the recovery of the parameters for real lenses. We also verified that LEMON has the potential to accelerate traditional modelling methods, by giving to the classical optimiser the LEMON predictions as starting points, resulting in a speed-up of up to 26 times the original time needed to model a sample of gravitational lenses, a result that would be impossible with randomly initialised guesses. Moreover, LEMON can be used to cross-validate results from the traditional modelling methods, and thus has the potential to reduce the failure rate of the Euclid modelling pipeline. This work represents a significant step towards efficient, automated gravitational lens modelling, which is crucial for handling the large data volumes expected from Euclid.

• Bisigello, L.; Rodighiero, G.; Fotopoulou, S.; Ricci, F.; Jahnke, K. & Feltre, A. [Show all 350 contributors for this article] (2026). Euclid Quick Data Release (Q1): XXIV. Extending the quest for little red dots to z < 4. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554537. Full text in Research Archive Show summary

  Recent observations with the James Webb Space Telescope (JWST) have revealed an interesting population of sources with a compact morphology and a characteristic v-shaped continuum, namely blue at a rest frame λ < 4000 Å and red at longer wavelengths. The nature of these sources, which are called little red dots (LRDs), is still highly debated because it is unclear whether they host active galactic nuclei (AGNs) and their number seems to drop drastically at z < 4. We took advantage of the 63 deg^2 covered by the Euclid Quick Data Release (Q1) to extend the search for LRDs to brighter magnitudes and lower redshifts than what was possible with JWST. This is fundamental for a broader view of the evolution of this peculiar galaxy population. The selection was performed by fitting the available photometric data (Euclid, the Spitzer Infrared Array Camera (IRAC), and ground-based griz data) with two power laws to retrieve the rest-frame optical and UV slopes consistently over a wide redshift range (i.e. z < 7.6). We then excluded extended objects and possible line emitters and inspected the data visually to remove any imaging artefacts. The final selection included 3341 LRD candidates from z = 0.33 to z = 3.6, 29 of which were also detected in IRAC. The resulting rest-frame UV luminosity function, in contrast with previous JWST studies, shows that the number density of LRD candidates increases from high redshift to z = 1.5–2.5 and decreases at even lower redshifts. The subsample of more robust LRD candidates that are also detected with IRAC show a weaker evolution, however, which is affected by low statistics and limited by the IRAC resolution. The comparison with previous quasar UV luminosity functions shows that LRDs are not the dominant AGN population at z < 4 and MUV < −21. Follow-up studies of these LRD candidates are pivotal to confirm their nature, probe their physical properties, and determine whether they are compatible with JWST sources because the different spatial resolution and wavelength coverage of Euclid and JWST might select different samples of compact sources.

• Copin, Y.; Fumana, M.; Mancini, C.; Appleton, P. N.; Chary, R. & Conseil, S. [Show all 350 contributors for this article] (2026). Euclid Quick Data Release (Q1): VI. From spectrograms to spectra: The SIR spectroscopic processing function. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554627. Full text in Research Archive Show summary

  The Euclid space mission aims to investigate the nature of dark energy and dark matter by mapping the large-scale structure of the Universe. A key component of Euclid’s observational strategy is slitless spectroscopy, which is conducted using the Near Infrared Spectrometer and Photometer (NISP). This technique enables the acquisition of large-scale spectroscopic data without the need for targeted apertures, thus allowing for precise redshift measurements of millions of galaxies. These data are essential for Euclid’s core science objectives, including the study of cosmic acceleration and the evolution of galaxy clustering, and will enable many non-cosmological investigations. This study presents the SIR processing function, which is responsible for processing slitless spectroscopic data from Euclid’s NISP instrument. The objective is to generate fully calibrated science-grade one-dimensional spectra in order to ensure high-quality spectroscopic data for cosmological or astrophysical analyses. The processing function relies on a source catalogue generated from photometric data, effectively corrects detector effects, subtracts cross-contaminations, minimises self-contamination, calibrates wavelength and flux, and produces reliable spectra for later scientific use. The first Quick Data Release (Q1) of Euclid’s spectroscopic data provides approximately three million validated spectra for sources observed in the red-grism mode from a selected portion of the Euclid Wide Survey. We find that the wavelength accuracy and measured resolving power are within top-level mission requirements, thanks to the excellent optical quality of the instrument. The SIR processing function represents a significant step in processing slitless spectroscopic data for the Euclid mission. As the survey progresses, continued refinements and additional features will enhance its capabilities, thus further supporting high-precision cosmological and astrophysical measurements.

• Walmsley, M.; Holloway, P.; Lines, N. E.P.; Rojas, K.; Collett, T. E. & Verma, A. [Show all 374 contributors for this article] (2026). Euclid Quick Data Release (Q1): XXVI. The Strong Lensing Discovery Engine A – System overview and first lens sample. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202555141. Full text in Research Archive Show summary

  We present a catalogue of 497 galaxy-galaxy strong lenses in the Euclid Quick Release 1 data (63 deg^2). In the initial 0.45% of Euclid’s surveys, we double the total number of known lens candidates with space-based imaging. Our catalogue includes 250 grade A candidates, the vast majority of which (243) were previously unpublished. Euclid’s resolution reveals rare lens configurations of scientific value, including double-source-plane lenses, edge-on lenses, complete Einstein rings, and quadruply imaged lenses. We resolve lenses with small Einstein radii (θ_E < 1 arcsec) in large numbers for the first time. These lenses were found through an initial sweep by deep learning models followed by Space Warps citizen scientist inspection, expert vetting, and system-by-system modelling. Our search approach scales straightforwardly to Euclid Data Release 1 and, without changes, would yield approximately 7000 high-confidence (grade A or B) lens candidates by late 2026. Further extrapolating to the complete Euclid Wide Survey implies a likely yield of over 100 000 high-confidence candidates, transforming strong lensing science.

• Li, T.; Collett, T. E.; Walmsley, M.; Lines, N. E.P.; Rojas, K. & Nightingale, J. W. [Show all 338 contributors for this article] (2026). Euclid Quick Data Release (Q1): XXIX. The strong-lensing discovery engine D: Double-source-plane lens candidates. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554543. Full text in Research Archive
• Marleau, F. R.; Habas, R.; Carollo, D.; Tortora, C.; Duc, P. A. & Sola, E. [Show all 309 contributors for this article] (2026). Euclid Quick Data Release (Q1): X. A census of dwarf galaxies across a range of distances and environments. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554547. Full text in Research Archive Show summary

  The Euclid Q1 fields were selected for calibration purposes in cosmology and are therefore relatively devoid of nearby galaxies. However, this is precisely what makes them interesting fields in which to search for dwarf galaxies in local density environments. We took advantage of the unprecedented depth, spatial resolution, and field of view of the Euclid Quick Release (Q1) to build a census of dwarf galaxies in these regions. We have identified dwarf galaxies in a representative sample of 25 contiguous tiles in the Euclid Deep Field North (EDF-N) covering an area of 14.25 deg^2. The dwarf galaxy candidates were identified using a semi-automatic detection method based on properties measured by the Euclid pipeline and released as part of the catalogue produced by the MERge Processing Function (MER PF) pipeline. A selection cut in surface brightness and magnitude was used to produce an initial dwarf candidate catalogue, and this was followed by a cut in morphology (removing background spirals) and I_E − H_E colour (removing red ellipticals). This catalogue was then visually classified to produce a final sample of dwarf candidates, including their morphology, number of nuclei, globular cluster (GC) richness, and presence of a blue compact centre. We identified 2674 dwarf candidates, corresponding to 188 dwarfs per square degree. The visual classification of the dwarfs reveals a slightly uneven morphological mix of 58% ellipticals and 42% irregulars, with very few potentially GC-rich (1.0%) and nucleated (4.0%) candidates but a noticeable fraction (6.9%) of dwarfs with blue compact centres. The distance distribution of 388 (15%) of the dwarf candidates with spectroscopic redshifts peaks at about 400 Mpc. Their stellar mass distribution confirms that our selection effectively identifies dwarfs while minimising contamination. The most prominent dwarf overdensities are dominated by dwarf ellipticals, while dwarf irregulars are more evenly distributed across the field of view. This work highlights Euclid’s remarkable ability to detect and characterise dwarf galaxies across diverse masses, distances, and environments.

• Holloway, P.; Verma, A.; Walmsley, M.; Marshall, P. J.; More, A. & Collett, T. E. [Show all 346 contributors for this article] (2026). Euclid Quick Data Release (Q1): XXX. The Strong Lensing Discovery Engine E – Ensemble classification of strong gravitational lenses: lessons for Data Release 1★. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554617. Full text in Research Archive Show summary

  The Euclid Wide Survey (EWS) is expected to identify in the order of 100 000 galaxy-galaxy strong lenses across 14 000deg2. The Euclid Quick Data Release (Q1) of 63.1deg2Euclid images provides an excellent opportunity to test our lens-finding ability, and to verify the anticipated lens frequency in the EWS. Following the Q1 data release, eight machine learning networks from five teams were applied to approximately one million images. This was followed by a citizen science inspection of a subset of around 100 000 images, of which 65% received high network scores, with the remainder randomly selected. The top scoring outputs were inspected by experts to establish confident (grade A), likely (grade B), possible (grade C), and unlikely lenses. In this paper we combine the citizen science and machine learning classifiers into an ensemble, demonstrating that a combined approach can produce a purer and more complete sample than the original individual classifiers. Using the expert-graded subset as ground truth, we find that this ensemble can provide a purity of 52 ± 2% (grade A/B lenses) with 50% completeness (for context, due to the rarity of lenses a random classifier would have a purity of 0.05% and the best machine learning network in this work achieved 7.3% purity for the same completeness). We discuss future lessons for the first major Euclid data release (DR1), where the big-data challenges will become more significant and will require analysing more than ∼300 million galaxies, and thus the time investment of both experts and citizens must be carefully managed.

• Brun, V. Le; Bethermin, M.; Moresco, M.; Vibert, D.; Vergani, D. & Surace, C. [Show all 345 contributors for this article] (2026). Euclid Quick Data Release (Q1): VII. Characteristics and limitations of the spectroscopic measurements. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554607. Full text in Research Archive Show summary

  The spectroscopy processing function (SPE PF) of the Euclid pipeline is dedicated to the automatic analysis of 1D spectra to determine redshifts, line fluxes, and spectral classifications. The first Euclid Quick Data Release (Q1) delivers these measurements for all H_E < 22.5 objects identified in the photometric survey. In this paper, we present an overview of the SPE PF algorithm and assess its performance by comparing its results with high-quality spectroscopic redshifts from the Dark Energy Spectroscopic Instrument (DESI) survey in the Euclid Deep Field North. Our findings highlight remarkable accuracy in successful redshift measurements, with a bias of less than 3 × 10^−5 in (z_SPE − z_DESI)/(1 + z_DESI) and a high precision of approximately 10−3. The majority of spectra have only a single spectral feature or none at all. To avoid spurious detections, whereby noise features are misinterpreted as lines or lines are misidentified, it is therefore essential to apply well-defined criteria on quantities such as the redshift probability, or the Hα flux, and signal-to-noise ratio. Using a well-tuned quality selection, we achieve an 89% redshift success rate in the target redshift range for cosmology (0.9 < z < 1.8), which is well covered by DESI for z < 1.6. Outside this range in which the Hα line is observable, redshift measurements are less reliable, except for sources showing specific spectral features (e.g. two bright lines or strong continuum). The classification based on spectroscopy alone is effective for galaxies (about 80% success rate), while it is less efficient for stars and quasars (< 60%). Ongoing refinements along the entire chain of PFs are expected to enhance both redshift measurements and spectral classification, allowing us to define the large and reliable sample required for cosmological analyses. Taking into account the planned evolution of the spectroscopic pipeline, partially based on the limitations identified in this paper, these results are encouraging for Euclid’s future galaxy clustering measurements, even though the requirements are not yet fulfilled.

• Margalef-Bentabol, B.; Wang, L.; Marca, A. La; Rodriguez-Gomez, V.; Humphrey, A. & Fotopoulou, S. [Show all 333 contributors for this article] (2026). Euclid Quick Data Release (Q1): XVIII. First Euclid statistical study of the active galactic nuclei contribution fraction. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554583. Full text in Research Archive Show summary

  Active galactic nuclei (AGN) are an important phase in galaxy evolution. However, they can be difficult to identify due to their varied observational signatures. Furthermore, to understand the impact of an AGN on its host galaxy, it is important to quantify the strength of the AGN with respect to the host galaxy. We developed a deep learning (DL) model to identify AGN in imaging data by deriving the contribution of the central point source. The model was trained with Euclidised mock galaxy images in which we artificially injected different levels of AGN, in the form of varying contributions of the point spread function (PSF). Our DL-based method can precisely and accurately recover the injected AGN contribution fraction, f_PSF, with a mean difference between the predicted and true f_PSF of −0.0078 and an overall root mean square error of 0.051. With this new method, we move beyond the simplistic AGN versus non-AGN classification and are able to precisely quantify the AGN contribution and study galaxy evolution across a continuous spectrum of AGN activity. We applied our method to a stellar-mass-limited sample (with M_* ≥ 10^9.8 M⊙ and 0.5 ≤ z ≤ 2.0 ) selected from the first Euclid quick data release (Q1) and, using a threshold of f_PSF > 0.2, we identified 61 432 ± 70 AGN over 63.1 deg^2 (9.8 ± 0.1% of our sample). We compared these DL-selected AGN with AGN selected in the X-ray, mid-infrared (MIR), and via optical spectroscopy and investigated their overlapping fractions depending on different thresholds on the PSF contribution. We find that the overlap increases with increasing X-ray or bolometric AGN luminosity. We observe a positive correlation between the luminosity in the IE filter of the AGN and the host galaxy stellar mass, suggesting that supermassive black holes (SMBHs) generally grow faster (in absolute terms, i.e. the luminosity of the PSF component is larger) in more massive galaxies. Moreover, the mean relative contribution of the AGN is higher in the quiescent galaxy population than in the star-forming population. In terms of absolute power, starburst galaxies, as well as the most massive galaxies (across the star-formation main sequence), tend to host the most luminous AGN, indicating concomitant assembly of the SMBH and the host galaxy.

• Rojas, K.; Collett, T. E.; Barroso, J. A. Acevedo; Nightingale, J. W.; Stern, D. & Moustakas, L. A. [Show all 339 contributors for this article] (2026). Euclid Quick Data Release (Q1): XXVII. The Strong Lensing Discovery Engine B – Early strong lens candidates from visual inspection of high-velocity dispersion galaxies. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554605. Full text in Research Archive Show summary

  We present a search for strong gravitational lenses in Euclid imaging with a high stellar velocity dispersion (σ_v > 180 km s^−1) reported by SDSS and DESI. We performed expert visual inspection and classification of 11 660 Euclid images. We discovered 38 grade A and 40 grade B candidate lenses, which is consistent with an expected sample of ∼32. Palomar spectroscopy confirmed 5 lens systems, while DESI spectra confirmed one system, provided ambiguous results for another, and helped to discard a third system. The Euclid automated lens modeler modelled 53 candidates, confirmed 38 as lenses, failed to model 9, and ruled out 6 grade B candidates. For the remaining 25 candidates, we were unable to gather additional information. More importantly, our classified non-lenses provide an excellent training set for machine-learning lens classifiers. We created high-fidelity simulations of Euclid lenses by painting realistic lensed sources behind the tagged (non-lens) luminous red galaxies. This training set is the foundation stone for the Euclid galaxy-galaxy strong-lensing discovery engine.

• Magliocchetti, M.; Marca, A. La; Bisigello, L.; Bondi, M.; Ricci, F. & Fotopoulou, S. [Show all 168 contributors for this article] (2026). Euclid: Quick Data Release (Q1) – The connection between galaxy close encounters and radio activity. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202557391. Full text in Research Archive Show summary

  Using the vast statistics provided by both Euclid and the LOFAR surveys, we present the first large-scale study of the connection among radio emission, its morphology, and the merging properties of the hosts of radio sources up to z ∼ 2. By dividing the radio sample into active galactic nuclei (AGNs) and star-forming galaxies, we find that radio-emitting AGNs show a clear preference for residing within galaxies undergoing a merging event. This is more significant for AGNs that present complex or extended radio emission; indeed, about half of them are associated with merging systems, while only ∼15% are hosted by a non-interacting galaxy. The observed trend is primarily driven by AGNs residing at z < 1, especially in the case of high – P_144 MHz > 10^24 W Hz−1 sr−1 – radio luminosities (∼56% in mergers versus ∼10% in non-mergers regardless of radio appearance). On the other hand, this preference seems to disappear at higher redshifts, where only bright AGNs with extended radio emission still prefer galaxies undergoing a merging event. The situation is reversed in the case of radio-emitting star-forming galaxies, which are preferentially associated with non-interacting systems. This is more significant as we move towards low-radio-luminosity/star-formation objects (P_144 MHz < 10^23 W Hz−1 sr−1) for which we find ∼40% in non-mergers versus ∼20% in mergers. These values hold regardless of redshift. We interpreted the above result for AGNs with their need to accrete outer gas from local encounters in order to trigger (radio) activity, especially in the case of extended radio emission such as hot spots and lobes. This is mostly observed at z < 1, since galaxies in the local Universe are more gas deprived than their higher redshift counterparts. Internal gas reservoirs instead seem sufficient to trigger star formation within the majority of galaxies, which indeed mostly appear as non-interacting systems at all redshifts probed.

• Walmsley, M.; Huertas-Company, M.; Quilley, L.; Masters, K. L.; Kruk, S. & Remmelgas, K. A. [Show all 355 contributors for this article] (2026). Euclid Quick Data Release (Q1): IX. First visual morphology catalogue. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202555140. Full text in Research Archive Show summary

  We present a detailed visual morphology catalogue for the Euclid Quick Release 1 (Q1). Our catalogue includes galaxy features such as bars, spiral arms, and ongoing mergers, for the 378 000 bright (I_E < 20.5) or extended (area ≥700 pixels) galaxies in Q1. The catalogue was created by finetuning the Zoobot galaxy foundation models on annotations from an intensive one-month campaign by Galaxy Zoo volunteers. Our measurements are fully automated, and hence fully scaleable. This catalogue is the first 0.4% of the approximately 100 million galaxies where Euclid will ultimately resolve detailed morphology.

• Stevens, G.; Fotopoulou, S.; Bremer, M. N.; Zatarain, T. Matamoro; Jahnke, K. & Margalef-Bentabol, B. [Show all 273 contributors for this article] (2026). Euclid Quick Data Release (Q1): XXI. Active galactic nuclei identification using diffusion-based inpainting of Euclid VIS images. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554612. Full text in Research Archive Show summary

  Light emission from galaxies exhibit diverse brightness profiles, influenced by factors such as galaxy type, structural features, and interactions with other galaxies. Elliptical galaxies feature more uniform light distributions, while spiral and irregular galaxies have complex, varied light profiles due to their structural heterogeneity and star-forming activity. In addition, galaxies with active galactic nuclei (AGN) feature intense, concentrated emission from gas accretion around supermassive black holes, superimposed on regular galactic light, while quasi-stellar objects (QSOs) represent extreme cases in which AGN emissions dominate their host galaxies. The challenge of identifying AGN and QSOs has been discussed many times in the literature, often requiring multi-wavelength observations. This paper introduces a novel approach to identify AGN and QSOs from a single image. Diffusion models have recently been developed in the machine-learning literature to generate realistic-looking images of everyday objects. Utilising the spatial resolving power of the Euclid VIS images, we created a diffusion model trained on one million sources, without using any source pre-selection or labels. The model learns to reconstruct light distributions of normal galaxies, since the population is dominated by them. We conditioned the prediction of the central light distribution by masking the central few pixels of each source and reconstructed the light according to the diffusion model. We further used this prediction to identify sources that deviate from this profile by examining the reconstruction error of the few central pixels regenerated in each source’s core. Our approach, solely using VIS imaging, features high completeness compared to traditional methods of AGN and QSO selection, including optical, near-infrared, mid-infrared, and X-rays. Our study offers practical insights for refining diffusion models and broadening their applications throughout the Euclid survey area, underscoring the utility of this approach in diverse astronomical contexts beyond just AGN identification.

• Tarsitano, F.; Fotopoulou, S.; Banerji, M.; Petley, J.; Faisst, A. L. & Tucci, M. [Show all 324 contributors for this article] (2026). Euclid Quick Data Release (Q1): XIX. First study of red quasars selection. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554591. Full text in Research Archive Show summary

  Red quasars constitute an important but elusive phase in the evolution of supermassive black holes, where dust obscuration can significantly alter their observed properties. They have broad emission lines, like other quasars, but their optical continuum emission is significantly reddened, which is why they were traditionally identified based on near- and mid-infrared selection criteria. This work showcases the capability of the Euclid space telescope to find a large sample of red quasars, using Euclid near infrared (NIR) photometry. We first conduct a forecast analysis, comparing a synthetic catalogue of red quasars with COSMOS2020. Using template fitting, we reconstruct Euclid-like photometry for the COSMOS sources and identify a sample of candidates in a multi-dimensional colour-colour space achieving 98% completeness for mock red quasars with 30% contaminants. To refine our selection function, we implement a probabilistic Random Forest classifier, and use UMAP visualisation to disentangle non-linear features in colour-space, reaching 98% completeness and 88% purity. A preliminary analysis of the candidates in the Euclid Deep Field Fornax (EDF-F) shows that, compared to VISTA+DECam-based colour selection criteria, Euclid’s superior depth, resolution, and optical-to-NIR coverage improves the identification of the reddest, most obscured sources. Notably, the Euclid exquisite resolution in the I_E filter unveils the presence of a candidate dual quasar system, highlighting the potential for this mission to contribute to future studies on the population of dual AGN. The resulting catalogue of candidates, including more the 150 000 sources, provides a first census of red quasars in Euclid Q1 and sets the groundwork for future studies in the Euclid Wide Survey (EWS), including spectral follow-up analyses and host morphology characterisation.

• Klein, M.; George, K.; Mohr, J. J.; Altieri, B.; Amendola, L. & Andreon, S. [Show all 147 contributors for this article] (2026). Euclid: Quick Data Release (Q1) – Watching ICM-selected galaxy clusters with Euclid eyes: Prospects of Euclid data in the context of large SZE- and X-ray-based surveys. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202556233. Full text in Research Archive Show summary

  Galaxy clusters detected through their X-ray emission or Sunyaev–Zeldovich effect (SZE), both produced by the intracluster medium (ICM), have been successfully used in cosmological and astrophysical studies. To maximise the scientific return and robustness of such studies, complementary information from other datasets is required. Systematic cluster confirmation and redshifts of ICM-selected cluster candidates are typically provided by wide-field optical and infrared imaging surveys, which are becoming increasingly challenged by ongoing ICM-selected samples. Particularly at high redshifts (z > 1) reached by future SZE-selected samples, current large surveys may not be sufficient for this task. Deep high-resolution infrared surveys, such as those conducted with Euclid, are therefore essential to confirm the majority of high-redshift clusters in these future samples. In this context, we present an analysis of the first sizeable Euclid dataset (Q1), which overlaps with several ICM-selected cluster samples. We applied an adaptation of the MCMF cluster redshift and confirmation tool to Euclid data to estimate key cluster properties, including redshift and richness, and to predict its capabilities to confirm high-redshift galaxy clusters. We find promising performance in redshift and richness estimation, particularly at high redshift. The performance in richness estimation at low redshifts (z < 0.4) is currently impacted by limitations of the Q1 dataset and are likely to improve in future data releases. By comparing MCMF measurements along random lines of sight with similar measurements from the SZE-based ACT-DR5 MCMF catalogue, we predict that the ability to confirm clusters at 1 < z < 2 using Euclid will be comparable to that of current large optical surveys at z < 0.6 and will significantly enhance the capability of cluster confirmation at high redshifts. SZE-selected cluster samples will therefore especially benefit from overlap with Euclid datasets. Through study of the five known high-z SZE-selected clusters in Q1, we identified the highest-redshift jellyfish galaxy candidate found to date in an ICM-selected cluster. This galaxy, EUCL J035330.86−504347.6, is located in the massive cluster SPT-CL J0353−5043 at z = 1.32. We also found two massive star-forming galaxies projected close to the core of ACT-CL J0350.0−4819 (z ≃ 1.46) and evidence of strong lensing features in SPT-CL J0353−5043 and SPT-CL J0421−4845.

• Žerjal, M.; Dominguez-Tagle, C.; Vitas, N.; Sedighi, N.; Martín, E. L. & Osorio, M. R. Zapatero [Show all 181 contributors for this article] (2026). Euclid: Quick Data Release (Q1) – A photometric search for ultracool dwarfs in the Euclid Deep Fields. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202556980. Full text in Research Archive Show summary

  We present a catalogue of 5306 new ultracool dwarf (UCD) candidates in the three Euclid Deep Fields in the Q1 data release. They range from late M to late T dwarfs, and include 1200 L and T dwarfs. A total of 546 objects have been spectroscopically confirmed, including 329 L dwarfs and 26 T dwarfs. We also provide empirical Euclid colours as a function of spectral type. Our UCD selection criteria are based only on colour (I_E − Y_E > 2.5). The combined requirement for optical detection and a stringent signal-to-noise ratio threshold ensure a high purity of the sample, but at the expense of completeness, especially for T dwarfs. The detections range from magnitudes 19 and 24 in the near-infrared bands, and extend down to 26 in the optical band. We discuss Euclid’s capability to identify UCD candidates based on its photometric passbands. The average surface density of detected UCDs on the sky is approximately 100 objects per deg^2, including 20 L and T dwarfs per deg^2. This leads to an expectation of at least 1.4 million UCDs in the final data release of the Euclid Wide Survey, including at least 300 000 L dwarfs, and more than 2600 T dwarfs, using the strict selection criteria from this work.

• Polenta, G.; Frailis, M.; Alavi, A.; Appleton, P. N.; Awad, P. & Bonchi, A. [Show all 369 contributors for this article] (2026). Euclid Quick Data Release (Q1): III. NIR processing and data products. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554657. Full text in Research Archive Show summary

  This paper describes the near-infrared processing function (NIR PF) that processes NIR images from the Near-Infrared Spectrometer and Photometer (NISP) instrument on board the Euclid satellite. NIR PF consists of three main components: (i) a common pre-processing stage for both photometric (NIR) and spectroscopic (SIR) data to remove instrumental effects; (ii) astrometric and photometric calibration of NIR data, along with catalogue extraction; and (iii) resampling and stacking. The necessary calibration products are generated using dedicated pipelines that process observations from both the early performance verification (PV) phase in 2023 and the nominal survey operations. After outlining the pipelineś structure and algorithms, we describe its application to Euclid Q1 images. For Q1, we achieved an astrometric accuracy of 5–8 mas, a relative photometric accuracy of 5 mmag, along with an absolute flux calibration limited by the 1% uncertainty of the Hubble Space Telescope (HST) CALSPEC database. We characterised the point spread function (PSF), which we found to be very stable across the focal plane. Finally, we discuss current limitations of NIR PF that will be improved upon for future data releases.

• Girardi, G.; Rodighiero, G.; Bisigello, L.; Enia, A.; Grazian, A. & Bontà, E. Dalla [Show all 328 contributors for this article] (2026). Euclid Quick Data Release (Q1): XXII. An investigation of optically faint, red objects in the Euclid Deep Fields. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554615. Full text in Research Archive Show summary

  Our understanding of cosmic star formation at z > 3 used to largely rely on rest-frame UV observations. However, these observations overlook dusty and massive sources, resulting in an incomplete census of early star-forming galaxies. Recent infrared data from Spitzer and the James Webb Space Telescope (JWST) have revealed a hidden population at z ∼ 3 − 6 with extreme red colours. Taking advantage of the overlap between imaging of the Euclid Deep Fields (EDFs), covering about 60 deg^2, and ancillary Spitzer observations, we identified 27 000 extremely red objects with H_E − IRAC2 > 2.25 (dubbed HIEROs) down to a 10σ completeness magnitude limit of IRAC2 = 22.5 AB. After a visual investigation to discard artefacts and any objects with troubling photometry, we were left with a final sample of 3900 candidates. We retrieved the physical parameter estimates for these objects from the spectral energy distribution-fitting tool CIGALE. Our results confirm that HIERO galaxies can populate the high-mass end of the stellar mass function at z > 3, with some sources reaching extreme stellar masses (M* > 10^11 M⊙) and exhibiting high dust attenuation values (A_V > 3). However, we consider the stellar mass estimates unreliable for sources at z > 3.5. For this reason, we favour a more conservative lower-z solution. The challenges faced by spectral energy distribution-fitting tools in accurately characterising these objects underscore the need for further studies that incorporate both observations at shorter wavelengths and spectroscopic data. Euclid spectra will help resolve degeneracies and better constrain the physical properties of the brightest galaxies. Given the extreme nature of this population, characterising these sources is crucial for building a comprehensive picture of galaxy evolution and stellar mass assembly across most of the history of the Universe. This work demonstrates Euclid’s potential to provide statistical samples of rare objects, such as massive, dust-obscured galaxies at z > 3, which will be prime targets for JWST and the Atacama Large Millimeter/submillimeter Array (ALMA).

• McCracken, H. J.; Benson, K.; Dolding, C.; Flanet, T.; Grenet, C. & Herent, O. [Show all 379 contributors for this article] (2026). Euclid Quick Data Release (Q1): II. VIS processing and data products. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554594. Full text in Research Archive Show summary

  This paper describes the VIS processing function (VIS PF) of the Euclid ground segment pipeline, which processes and calibrates raw data from the VIS camera. We present the algorithms used in each processing element along with a description of the on-orbit performance of VIS PF based on performance verification and Q1 datasets. We demonstrate that the principal performance metrics (image quality, astrometric accuracy, photometric calibration) are within pre-launch specifications. The image-to-image photometric scatter is less than 0.8% and absolute astrometric accuracy compared to Gaia is 5 mas. Image quality is stable over all Q1 images, with a full width at half maximum (FWHM) of 0.″16. The stacked images (combining four nominal and two short exposures) reach I_E = 25.6 (10σ, measured as the variance of 1.″3 diameter apertures). We also describe quality control metrics provided with each image, and an appendix provides a detailed description of the provided data products. The excellent quality of these images demonstrates the immense potential of Euclid VIS data for weak lensing. VIS data covering most of the extragalactic sky will provide a lasting high-resolution atlas of the Universe.

• Quilley, L.; Damjanov, I.; Lapparent, V. de; Paulino-Afonso, A.; Sánchez, H. Domínguez & Ferré-Mateu, A. [Show all 335 contributors for this article] (2026). Euclid Quick Data Release (Q1): VIII. Exploring galaxy morphology across cosmic time through Sérsic fits. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554585. Full text in Research Archive Show summary

  We present the results of the single-component Sérsic profile fitting for the magnitude-limited sample of IE < 23 galaxies within the 63.1 deg^2 area of the Euclid Quick Data Release (Q1). The associated morphological catalogue includes two sets of structural parameters fitted using SourceXtractor++: one for VIS I_E images and one for a combination of three NISP images in Y_E, J_E, and H_E bands. We compared the resulting Sérsic parameters to other morphological measurements provided in the Q1 data release and to the equivalent parameters based on higher-resolution Hubble Space Telescope imaging. These comparisons confirmed the consistency and the reliability of the fits to Q1 data. Our analysis of colour gradients shows that NISP profiles systematically have smaller effective radii (R_e) and larger Sérsic indices (n) than in VIS. In addition, we highlight trends in NISP-to-VIS parameter ratios with both magnitude and nVIS. From the 2D bimodality of the (u − r) colour-log(n) plane, we defined a (u − r)_lim(n) that separates early- and late-type galaxies (ETGs and LTGs). We used the two sub-populations to examine the variations of n across well-known scaling relations at z < 1. The ETGs display a steeper size–stellar mass relation than the LTGs, indicating a difference in the main drivers of their mass assembly. Similarly, LTGs and ETGs occupy different parts of the stellar mass–star-formation rate plane, with ETGs at higher masses than LTGs and further below the main sequence of star-forming galaxies. This clear separation highlights the link known between the shutdown of star formation and morphological transformations in the Euclid imaging data set. In conclusion, our analysis demonstrates both the robustness of the Sérsic fits available in the Q1 morphological catalogue and the wealth of information they provide for studies of galaxy evolution with Euclid.

• Fabricius, M.; Saglia, R.; Balzer, F.; Ecker, L. R.; Thomas, J. & Bender, R. [Show all 169 contributors for this article] (2026). Euclid: Quick Data Release (Q1) – Secondary nuclei in early-type galaxies. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202557217. Full text in Research Archive Show summary

  Massive early-type galaxies (ETGs; M > 10^11 M⊙) are believed to form primarily through mergers of less massive progenitors, which leave behind numerous traces of violent formation histories, such as stellar streams and shells. A particularly striking signature of these mergers is the formation of supermassive black hole (SMBH) binaries, which can create depleted stellar cores through interactions with stars on radial orbits – a process known as core scouring. The secondary SMBH in such systems may still carry a dense stellar envelope and thereby remain observable for some time as a secondary nucleus while it sinks towards the shared gravitational potential of the merged galaxy. Direct observations of secondary nuclei on sub-kiloparsec scales remain rare, with only a few notable cases, such as NGC 5419. Investigating such features and building up statistics requires both high spatial resolution and wide-field coverage, a capability uniquely provided by Euclid. In this study, we leverage Euclid’s Q1 Early Release data to systematically search for secondary nuclei in ETGs. We present a preliminary sample of 666 candidate systems distributed over 504 hosts (some of which contain multiple secondary nuclei). The vast majority of these fall at separations of 3 kpc to 15 kpc, indicative of normal mergers. However, 44 fall at projected separations of less than 2 kpc. We argue that this most interesting subset of secondary nucleus candidates – those at very close angular separations – are unlikely to be a consequence of chance alignments. We show that their stellar masses are mostly too large for them to be globular clusters and that a significant subset are unresolved even at Euclid’s spatial resolution, rendering them too small to be dwarf galaxies. These objects may represent the highest-density nuclei of a previously merged galaxy currently sinking into the centre of the new common gravitational potential, and thus they likely host a secondary SMBH. We also demonstrate that convolutional neural networks offer a viable avenue to detect multiple nuclei in the 30 times larger sky coverage of the future Euclid DR1. Finally, we argue that our method can detect the remnants of a recoil event from two merged SMBHs, as two of our secondary nuclei candidates are unresolved at the Euclid spatial resolution, appear at projected physical separations of less than 2 kpc, and appear in hosts of M > 10^11 M⊙, which makes them viable candidates.

• Huertas-Company, M.; Walmsley, M.; Siudek, M.; Iglesias-Navarro, P.; Knapen, J. H. & Serjeant, S. [Show all 338 contributors for this article] (2026). Euclid Quick Data Release (Q1): XI. A first look at the fraction of bars in massive galaxies at z < 1. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554606. Full text in Research Archive Show summary

  Stellar bars are key structures in disc galaxies, driving angular momentum redistribution and influencing processes such as bulge growth and star formation. Quantifying the bar fraction as a function of redshift and stellar mass is therefore important for constraining the physical processes that drive disc formation and evolution across the history of the Universe. Leveraging the unprecedented resolution and survey area of the Euclid Q1 data release, combined with the Zoobot deep-learning model trained on citizen science labels, we identified 7711 barred galaxies with M* ≳ 10^10 M_⊙ in a magnitude-selected sample (IE < 20.5) spanning 63.1 deg^2. We measured a mean bar fraction of 0.2–0.4, consistent with previous studies. At fixed redshift, massive galaxies exhibit higher bar fractions, while lower-mass systems show a steeper decline with redshift, suggesting earlier disc assembly in massive galaxies. Comparisons with cosmological simulations (e.g. TNG50, Auriga) reveal a broadly consistent bar fraction, but highlight overpredictions for high-mass systems, pointing to potential over-efficiency in central stellar mass build-up in simulations. A semi-empirical model (Decode) in which bar formation is regulated by the gas fraction broadly reproduces the observed bar fractions. These findings demonstrate the transformative potential of Euclid for galaxy morphology studies and underscore the importance of refining theoretical models to better reproduce observed trends. Future work will explore finer mass bins, environmental correlations, and additional morphological indicators.

• Enia, A.; Pozzetti, L.; Bolzonella, M.; Bisigello, L.; Hartley, W. G. & Saulder, C. [Show all 350 contributors for this article] (2026). Euclid Quick Data Release (Q1): XIV. A first view of the star-forming main sequence in the Euclid Deep Fields. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554576. Full text in Research Archive Show summary

  The star-forming main sequence (SFMS) is a tight relation observed between stellar masses and star formation rates (SFR) in a population of galaxies. This relation is observed at different redshifts, in various morphological, and environmental domains, and is key to understanding the underlying relations between a galaxy budget of cold gas and its stellar content. Euclid Quick Data Release 1 (Q1) gives us the opportunity to investigate this fundamental relation in galaxy formation and evolution. We complement the Euclid release with public IRAC observations of the Euclid Deep Fields, improving the quality of recovered photometric redshifts, stellar masses, and SFRs, as is shown both with simulations and a comparison with available spectroscopic redshifts. From Q1 data alone, we recover more than ∼30 k galaxies with log_10(M_*/M_⊙) > 11, giving a precise constraint of the SFMS at the high-mass end. We investigated the SFMS, in a redshift interval between 0.2 and 3.0, comparing our results with the existing literature and fitting them with a parameterisation taking into account the presence of a bending of the relation at the high-mass end, depending on the bending mass, M0. We find good agreement with previous results in terms of M0 values, and an increasing trend for the relation scatter at higher stellar masses. We also investigate the distribution of physical (e.g. dust absorption, AV, and formation age) and morphological properties (e.g., Sérsic index and radius) in the SFR–stellar mass plane, and their relation with the SFMS. These results highlight the potential of Euclid in studying the fundamental scaling relations that regulate galaxy formation and evolution in anticipation of the forthcoming Data Release 1.

• Romelli, E.; Kümmel, M.; Dole, H.; Gracia-Carpio, J.; Merlin, E. & Galeotta, S. [Show all 347 contributors for this article] (2026). Euclid Quick Data Release (Q1): IV. From images to multi-wavelength catalogues: The Euclid MERge processing function. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554586. Full text in Research Archive Show summary

  The Euclid satellite is an ESA mission that was launched in July 2023. Euclid is working in its regular observing mode with the target of observing an area of 14 000 deg^2 with two instruments, the Visible Camera (VIS) and the Near IR Spectrometer and Photometer (NISP) down to I_E = 24.5 mag (10 σ) in the Euclid Wide Survey. Ground-based imaging data in the ugriz bands complement the Euclid data to enable photo-z determination and VIS PSF modelling for weak lensing analysis. Euclid investigates the distance-redshift relation and the evolution of cosmic structures by measuring the shapes and redshifts of galaxies and clusters of galaxies out to z ∼ 2. Generating the multi-wavelength catalogues from Euclid and ground-based data is an essential part of the Euclid data processing system. In the framework of the Euclid Science Ground Segment (SGS), the aim of the MERge Processing Function (MER PF) pipeline is to detect objects in the Euclid imaging data, measure their properties, and merge them into a single multi-wavelength catalogue. The MER PF pipeline performs source detection on both visible (VIS) and near-infrared (NIR) images and offers four different photometric measurements: Kron total flux, aperture photometry on PSF-matched images, template fitting photometry, and Sérsic fitting photometry. Furthermore, the MER PF pipeline measures a set of ancillary quantities, spanning from morphology to quality flags, to better characterise all detected sources. In this paper, we show how the MER PF pipeline is designed, detailing its main steps, and we show that the pipeline products meet the tight requirements that Euclid aims to achieve on photometric accuracy. We also present the other measurements (e.g., morphology) that are included in the OU-MER output catalogues and we list all output products coming out of the MER PF pipeline.

• Gouin, C.; Laigle, C.; Sarron, F.; Bonnaire, T.; Sorce, J. G. & Aghanim, N. [Show all 340 contributors for this article] (2026). Euclid Quick Data Release (Q1): XXXV. The role of cosmic connectivity in shaping galaxy clusters. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554655. Full text in Research Archive Show summary

  The matter around galaxy clusters is distributed over several filaments, reflecting their positions as nodes in the large-scale cosmic web. The number of filaments connected to a cluster, i.e. its connectivity, is expected to affect the physical properties of clusters. Using the first Euclid galaxy catalogue from the Euclid Quick Release 1 (Q1), we investigated the connectivity of galaxy clusters and how it correlates with their physical and galaxy member properties. Around 220 clusters located within the three fields of Q1 (covering ∼63 deg2) were analysed in the redshift range 0.2  <  z  <  0.7. Due to the photometric redshift uncertainty, we reconstructed the cosmic web skeleton, and measured the cluster connectivity, in 2D projected slices with a thickness of 170 comoving h−1 Mpc and centred on each cluster redshift, by using two different filament finder algorithms on the most massive galaxies (M★ > 1010.3 M⊙). In agreement with previous measurements, we recovered the mass-connectivity relation independently of the filament detection algorithm, showing that the most massive clusters are, on average, connected to a larger number of cosmic filaments, consistent with hierarchical structure formation models. Furthermore, we explored the possible correlations between connectivities and two cluster properties: the fraction of early-type galaxies and the Sérsic index of galaxy members. Our result suggests that the clusters populated by early-type galaxies exhibit higher connectivity compared to clusters dominated by late-type galaxies. These preliminary investigations highlight our ability to quantify the impact of the cosmic web’s connectivity on cluster properties with Euclid.

• Siudek, M.; Huertas-Company, M.; Smith, M. J.; Martínez-Solaeche, G.; Lanusse, F. & Ho, S. [Show all 323 contributors for this article] (2026). Euclid Quick Data Release (Q1): XIII. Exploring galaxy properties with a multi-modal foundation model. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554611. Full text in Research Archive Show summary

  Modern astronomical surveys, such as the Euclid mission, produce high-dimensional, multi-modal datasets that include imaging and spectroscopic information for millions of galaxies. These data serve as an ideal benchmark for large, pre-trained multi-modal models, which can leverage vast amounts of unlabelled data. In this work, we present the first exploration of Euclid data with AstroPT, an autoregressive multi-modal foundation model trained on approximately 300000 optical and infrared Euclid images and spectral energy distributions (SEDs) from the first Euclid Quick Data Release. We compare self-supervised pre-training with baseline fully supervised training across several tasks: galaxy morphology classification; redshift estimation; similarity searches; and outlier detection. Our results show that: (a) AstroPT embeddings are highly informative, correlating with morphology and effectively isolating outliers; (b) including infrared data helps to isolate stars, but degrades the identification of edge-on galaxies, which are better captured by optical images; (c) simple fine-tuning of these embeddings for photometric redshift and stellar mass estimation outperforms a fully supervised approach, even when using only 1% of the training labels; and (d) incorporating SED data into AstroPT via a straightforward multi-modal token-chaining method improves photo-z predictions, and allow us to identify potentially more interesting anomalies (such as ringed or interacting galaxies) compared to a model pre-trained solely on imaging data.

• Tucci, M.; Paltani, S.; Hartley, W. G.; Dubath, F.; Morisset, N. & Bolzonella, M. [Show all 346 contributors for this article] (2026). Euclid Quick Data Release (Q1): V. Photometric redshifts and physical properties of galaxies through the PHZ processing function. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554588. Full text in Research Archive Show summary

  The ESA Euclid mission will measure the photometric redshifts of billions of galaxies to provide an accurate 3D view of the Universe at optical and near-infrared wavelengths. Photometric redshifts are determined by the PHotometric redshift (PHZ) processing function on the basis of the multi-wavelength photometry of Euclid and ground-based observations. In this paper, we describe in detail the so-called PHZ processing used for the first ‘quick’ (Q1) Euclid data release, along with the output products and their validation with respect to the Euclid requirements. The PHZ pipeline is responsible for the following main tasks: i) source classification into star, galaxy, and quasar (or QSO) classes based on photometric colours; ii) determination of photometric redshifts for the core science; and iii) determination of physical properties of galaxies for non-cosmological science. The classification is able to provide a star sample with a high level of purity, a highly complete galaxy sample, and reliable probabilities of belonging to those classes. The identification of QSOs is instead more problematic: photometric information available in the Euclid Wide Survey alone seems to be insufficient to accurately separate QSOs from galaxies. The performance of the pipeline in the determination of photometric redshifts has been tested using the COSMOS2020 catalogue and a large sample of spectroscopic redshifts. The results in both cases are in line with expectations: the precision of the estimates are compatible with Euclid requirements; however, as expected, a bias correction is needed to achieve the accuracy level required for the cosmological probes. Finally, the pipeline provides reliable estimates of the physical properties of galaxies, in good agreement with findings from the COSMOS2020 catalogue – apart from an unrealistically large fraction of very young galaxies with very high specific star-formation rates. However, the application of appropriate priors is sufficient to obtain reliable physical properties for those problematic objects. We present several areas for improvement for future Euclid data releases.

• Gentile, F.; Daddi, E.; Elbaz, D.; Enia, A.; Magnelli, B. & Billand, J. B. [Show all 324 contributors for this article] (2026). Euclid Quick Data Release (Q1): XII. Quenching precedes bulge formation in dense environments but follows it in the field. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202557633. Full text in Research Archive Show summary

  The well-known bimodality between star-forming discs and quiescent spheroids requires the existence of two main processes: galaxy quenching, causing the strong reduction of star formation, and morphological transformation, causing the transition from disc-dominated structures to bulge-dominated ones. In this paper, we aim to understand the link between these two processes and their relation with the stellar mass of galaxies and their local environment. Taking advantage of the first data released by the Euclid Collaboration, covering more than 60 deg^2 with space-based imaging and photometry, we analyse a mass-complete sample of nearly one million galaxies in the range 0.25 < z < 1 with M_* > 10^9.5 M_⊙, using a combination of photometric and spectroscopic redshifts. We divide the sample into four sub-populations of galaxies, based on their star-formation activity (star-forming and quiescent) and morphology (disc-dominated and bulge-dominated). We then analyse the physical properties of these populations and their relative abundances in the stellar mass versus local density plane. Together with confirming the passivity-density relation and the morphology-density relation, we find that quiescent discy galaxies are more abundant in the low-mass regime of high-density environment where log_10(1 + δ) > 1.3. At the same time, star-forming bulge-dominated galaxies are more common in field regions with log_10(1 + δ) < 0.8, preferentially at high masses. Building on these results and interpreting them through comparison with simulations, we propose a scenario where the evolution of galaxies in the field significantly differs from that in higher-density environments. The morphological transformation in the majority of field galaxies takes place before the onset of quenching and is mainly driven by secular processes taking place within the main sequence, leading to the formation of star-forming bulge-dominated galaxies as intermediate-stage galaxies. Conversely, quenching of star formation precedes morphological transformation for most galaxies in higher-density environments. This causes the formation of quiescent disc-dominated galaxies before their transition into bulge-dominated ones.

• Andrews, A.; Jasche, J.; Lavaux, G.; Leclercq, F.; Finelli, F. & Akrami, Y. [Show all 150 contributors for this article] (2026). Euclid: Field-level inference of primordial non-Gaussianity and cosmic initial conditions. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202553802. Full text in Research Archive Show summary

  A primary target of the Euclid space mission is to constrain early-universe physics by searching for deviations from a primordial Gaussian random field. A significant detection of primordial non-Gaussianity would rule out the simplest models of cosmic inflation and transform our understanding of the origin of the Universe. This paper forecasts how well field-level inference of galaxy redshift surveys can constrain the amplitude of local primordial non-Gaussianity, f_NL(local), within a Bayesian hierarchical framework, in the upcoming Euclid data. We designed and simulated mock datasets and performed Markov chain Monte Carlo analyses using a full-field forward modelling approach. By including the formation history of the cosmic matter field in the analysis, the method takes into account all available probes of primordial non-Gaussianity, and goes beyond statistical summary estimators of f_NL(local). Probes include, for example, two-point and higher-order statistics, peculiar velocity fields, and scale-dependent galaxy biases. Furthermore, the method simultaneously handles systematic survey effects, such as selection effects, survey geometries, and galaxy biases. The forecast shows that, using simulated Euclid data, the method can achieve a precision of σ(f_NL(local)) = 2.6 (68.3% confidence level), assuming a grid resolution of ΔL = 31.25 h^−1 Mpc and a cut-off scale of k_NF = 0.1 h Mpc^−1. We also provide data products, including realistic simulations with non-zero values of f_NL(local) and maps of adiabatic curvature fluctuations. The results underscore the feasibility and advantages of field-level inference to constrain f_NL(local) in galaxy redshift surveys. Our approach consistently captures all the information available in the large-scale structure to constrain f_NL(local), and resolves the degeneracy between early-universe physics and late-time gravitational effects, while mitigating the impact of systematic and observational effects.

• Cleland, C.; Mei, S.; Lucia, G. De; Fontanot, F.; Fu, H. & Lovell, C. C. [Show all 323 contributors for this article] (2026). Euclid Quick Data Release (Q1): XXXIX. The evolution of the passive-density and morphology-density relations between z = 0.25 and z = 1. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554581.
• Fu, Y.; Bouwens, R.; Caputi, K. I.; Vergani, D.; Scialpi, M. & Margalef-Bentabol, B. [Show all 357 contributors for this article] (2026). Euclid Quick Data Release (Q1): XXXVIII. Euclid spectroscopy of quasars. 1. Identification and redshift determination of 3500 bright quasars. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202558490.
• Burger, P. A.; Aricò, G.; Linke, L.; Angulo, R. E.; Broxterman, J. C. & Schaye, J. [Show all 156 contributors for this article] (2026). Euclid: An emulator for baryonic effects on the matter bispectrum. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 705. doi: 10.1051/0004-6361/202556061. Full text in Research Archive
• Graet, J. Le; Secroun, A.; Tourneur-Silvain, M.; Gillard, W.; Fourmanoit, N. & Escoffier, S. [Show all 124 contributors for this article] (2026). Euclid: methodology for derivation of IPC-corrected conversion gain of nonlinear CMOS APS. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 705. doi: 10.1051/0004-6361/202556173. Full text in Research Archive Show summary

  We introduce a fast method to measure the conversion gain in complementary metal-oxide-semiconductor active pixel sensors, which accounts for nonlinearity and interpixel capacitance (IPC). The standard “mean-variance” method is biased because it assumes that pixel values depend linearly on the signal, and existing methods to correct for nonlinearity still introduce significant biases. While current IPC correction methods are prohibitively slow for a per-pixel application, our new method uses separate measurements of the IPC kernel to calculate the gain almost instantaneously. Using test data from a flight detector of the ESA Euclid mission, the IPC correction recovers the results of slower methods with 0.1% accuracy. The nonlinearity correction ensures that the estimated gain is independent of signal, correcting a bias of more than 2.5%.

• Howell, J. M.; Ferguson, A. M.N.; Larsen, S. S.; Lançon, A.; Annibali, F. & Cuillandre, J. C. [Show all 152 contributors for this article] (2026). Euclid: Early Release Observations - The star cluster systems of the Local Group dwarf galaxies IC 10 and NGC 6822. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 706. doi: 10.1051/0004-6361/202556953. Full text in Research Archive Show summary

  Star clusters are valuable indicators of galaxy evolution, offering insights into the buildup of stellar populations across cosmic time. Understanding the intrinsic star cluster populations of dwarf galaxies is particularly important given these systems’ role in the hierarchical growth of larger systems. We use data from Euclid’s Early Release Observation programme to study star clusters in two star-forming dwarf irregular galaxies in the Local Group, NGC 6822 and IC 10 [M⋆ ~ (1–4) × 10^8 M⊙]. With Euclid, star clusters are resolved into individual stars across the main bodies and haloes of both galaxies. Through visual inspection of the IE images, we uncover 30 new star cluster candidates in NGC 6822 and 16 in IC 10, ranging from compact to diffuse extended clusters. We compile and re-evaluate previously identified literature candidates, resulting in final combined catalogues of 52 (NGC 6822) and 71 (IC 10) cluster candidates with confidence-based classifications. We present homogeneous photometry in IE, YE, JE, and HE, and in archival UBVRI data, alongside size measurements and properties derived from the spectral energy distribution fitting code BAGPIPES. Through synthetic cluster injection, we conclude our sample is ~50% complete to M ≲ 10^3 M⊙ for ages ≲100 Myr, and to M ≲ 2 × 10^4 M⊙ for ages of ~10 Gyr. We find that IC 10 has more young clusters than NGC 6822, and its young clusters extend to higher masses, consistent with its starburst nature. We find several old massive (≳10^5 M⊙) clusters in both dwarfs, including an exceptional cluster in NGC 6822’s outskirts with a mass of 1.3 × 10^6 M⊙, nearly twice as massive as any other old cluster in either galaxy. In NGC 6822, we also identify a previously undetected, old, and extended cluster (Rh = 12.4 ± 0.11 pc). Using well-defined criteria, we identify 11 candidate GCs in NGC 6822 and nine in IC 10. Both galaxies have high specific frequencies (SN) for their luminosities but remain consistent with the known GC scaling relationships in the low-luminosity regime.

• Duffy, C.; Hook, I. M.; Gutierrez, C. M.; Paterson, K.; Petrecca, V. & Moriya, T. J. [Show all 153 contributors for this article] (2026). Euclid: an automated system to match Rubin transient alerts to Euclid observations. RAS Techniques and Instruments (RASTI). 5. doi: 10.1093/rasti/rzag014. Full text in Research Archive Show summary

  The Vera C. Rubin observatory is expected to produce 10 million transient alerts per night in ugrizy filters, whilst Euclid is a visible to near-infrared space telescope engaged in a wide field survey. We present a prototype system to automatically match the transient alerts from Rubin to Euclid observations. The system produces joint light curves containing both visible and near-infrared photometry, and joint image cutouts. Using Zwicky Transient Facility alerts as a proxy for Rubin, we demonstrate the system in use in cases where Euclid did and did not detect the transient and highlight the value that can be added in each case. For transients detected by Euclid these benefits include identifying the supernovae (SNe) in observations taken prior to ground-based detection, thereby better constraining the explosion time, such as SN 2024pvw detected ~3 d prior to ground based detections. In cases where Euclid did not detect the transient, we demonstrate the benefit of adding Euclid observations to improve host morphology measurements and associations.

• Casas, S.; Cardone, V. F.; Sapone, D.; Frusciante, N.; Pace, F. & Parimbelli, G. [Show all 130 contributors for this article] (2026). Euclid: Constraints on f(R) cosmologies from the spectroscopic and photometric primary probes. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 707. doi: 10.1051/0004-6361/202348713. Full text in Research Archive Show summary

  We forecast the constraints that the Euclid mission will place on the Hu–Sawicki f(R) modified gravity model using galaxy clustering and weak lensing observations. Euclid’s primary probes will provide spectroscopic redshifts, photometric angular clustering, and weak lensing cosmic shear, thus allowing for precise tests of deviations from general relativity. We consider these observables to evaluate how well Euclid can constrain the extended model parameter fR0. For a fiducial value of |fR0| = 5 × 10^−6, we find that in our baseline pessimistic setting, Euclid will constrain log10|fR0| at the 4% level with spectroscopic clustering, at 2.7% with the cross-correlation of photometric probes, and at 1.8% when combining all primary probes. This corresponds to an estimation on this model parameter of approximately |fR0| = (5.0^+1.2_−0.9 × 10−6) at the 1σ level. We also forecast constraints for models with |fR0| = 5 × 10^−5 and |fR0| = 5 × 10^−7, finding that Euclid will distinguish these from the standard cosmological model at more than 3σ when using the full combination of primary probes. Euclid will be a powerful experiment to test modifications to gravity, provided that the theoretical systematics of the non-linear modelling are kept under control.

• Vinciguerra, S.; Bouchè, F.; Martinet, N.; Castiblanco, L.; Uhlemann, C. & Pires, S. [Show all 286 contributors for this article] (2026). Euclid preparation: LXXXV. Toward a DR1 application of higher-order weak lensing statistics. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 707. doi: 10.1051/0004-6361/202557573. Full text in Research Archive Show summary

  This is the second paper in the HOWLS (higher-order weak lensing statistics) series exploring the usage of non-Gaussian statistics for cosmology inference within Euclid. With respect to our first paper, we develop a full tomographic analysis based on realistic photometric redshifts that allows us to derive Fisher forecasts in the (σ8, w0) plane for a Euclid-like data release 1 (DR1) setup. We find that the five higher-order statistics (HOS) that satisfy the Gaussian likelihood assumption of the Fisher formalism (one-point probability distribution function, ℓ1-norm, peak counts, Minkowski functionals, and Betti numbers) each outperform the shear two-point correlation functions by a factor of 2.5 on the w0 forecasts, with only marginal improvement when used in combination with two-point estimators, suggesting that every HOS is able to retrieve both the non-Gaussian and Gaussian information of the matter density field. The similar performance of the different estimators is explained by a homogeneous use of multi-scale and tomographic information, optimized to lower computational costs. These results hold for the three mass mapping techniques of the Euclid pipeline, aperture mass, Kaiser–Squires, and Kaiser–Squires plus, and they are unaffected by the application of realistic star masks. Finally, we explored the use of HOS with the Bernardeau–Nishimichi–Taruya (BNT) nulling scheme approach, finding promising results toward applying physical scale cuts to HOS.

• Paterson, K.; Schirmer, M.; Okumura, K.; Venemans, B.; Jahnke, K. & Aghanim, N. [Show all 311 contributors for this article] (2026). Euclid: VI. NISP-P optical ghosts. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 707. doi: 10.1051/0004-6361/202555106. Full text in Research Archive Show summary

  The Near-Infrared Spectrometer and Photometer (NISP) on board Euclid includes several optical elements in its path that introduce artefacts into the data from non-nominal light paths. To ensure uncontaminated source photometry, these artefacts must be accurately accounted for. This paper focuses on two specific optical features in NISP’s photometric data (NISP-P): ghosts caused by the telescope’s dichroic beamsplitter, and the bandpass filters within the NISP fore-optics. Both ghost types exhibit a characteristic morphology and are offset from the originating stars. The offsets are well modelled using 2D polynomials; only stars brighter than approximately 10 magnitudes in each filter produce significant ghost contributions. The masking radii for these ghosts depend on both the source-star brightness and the filter wavelength, ranging from 20 to 40 pixels. We present the final relations and models used in the near-infrared (NIR) processing function (PF) to mask these ghosts for Euclid’s Quick Data Release (Q1).

• Varadaraj, R. G.; Bowler, R. A.A.; Jarvis, M. J.; Weaver, J. R.; Bañados, E. & Holloway, P. [Show all 166 contributors for this article] (2026). Euclid: Discovery of bright z ≃ 7 Lyman-break galaxies in UltraVISTA and Euclid COSMOS. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 707. doi: 10.1051/0004-6361/202557508. Full text in Research Archive Show summary

  We present a search for z ≃ 7 Lyman-break galaxies using the 1.72 deg^2 near-infrared (NIR) UltraVISTA survey in the COSMOS field, reaching 5σ depths in Y of 26.2. We incorporated deep Euclid optical and Euclid + Spitzer NIR imaging for a full spectral energy distribution (SED) fitting analysis. We found 289 candidate galaxies at 6.5 ≤ z ≤ 7.5 covering −22.6 ≤ MUV ≤ −20.2, faint enough to overlap with Hubble Space Telescope studies. We conducted a separate selection by including complementary Euclid performance verification imaging (reaching 5σ depths of 26.3), yielding 140 galaxies in 0.65 deg2, with 38 sources unique to this sample. We computed the rest-frame UV luminosity function (UV LF) from our samples, extending below the knee (M∗ = 21.14+0.28−0.25). We find that the shape of the UV LF is consistent with both a Schechter function and a double power law (DPL) at the magnitudes probed by this sample, with a DPL preferred at MUV < −22.5 when bright-end results are included. The UltraVISTA + Euclid sample provides a clean measurement of the LF due to the overlapping NIR filters identifying molecular absorption features in the SEDs of ultra-cool dwarf interlopers, and additional faint galaxies were recovered. A comparison with JWST LFs at z > 7 suggests a gentle evolution in the bright-end slope, although this is limited by a lack of robust bright-end measurements at z > 9. We forecast that in the Euclid Deep Fields, the removal of contaminant ultra-cool dwarfs as point sources will be possible at J_E < 24.5. Finally, we present a high-equivalent-width Lyman-α emitter candidate identified by combining HSC, VISTA, and Euclid broadband photometry, highlighting the synergistic power these instruments will have in the Euclid Auxiliary Fields for identifying extreme sources in the epoch of reionisation.

• Gillard, W.; Maciaszek, T.; Prieto, E.; Grupp, F.; Costille, A. & Jahnke, K. [Show all 358 contributors for this article] (2026). Euclid preparation: LXXVII. The NISP spectroscopy channel: Ground performance and calibration. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 707. doi: 10.1051/0004-6361/202555859. Full text in Research Archive Show summary

  ESA’s Euclid cosmology mission relies on the very sensitive and accurately calibrated spectroscopy channel of the Near-Infrared Spectrometer and Photometer (NISP). With three operational grisms in two wavelength intervals, NISP provides diffraction-limited slitless spectroscopy over a field of 0.57 deg2. A blue grism, BG_E, covers the wavelength range 926–1366 nm at a spectral resolution (ℛ) of 440–900 for a 0.″5 diameter source with a dispersion of 1.24 nm px^−1. Two red grisms, RGE, span 1206 to 1892 nm at ℛ = 550–740 and a dispersion of 1.37 nm px^−1. We describe the construction of the grisms as well as the ground testing of the flight model of the NISP instrument, where these properties were established.

• Diego, J. M.; Congedo, G.; Gavazzi, R.; Schrabback, T.; Atek, H. & Jain, B. [Show all 186 contributors for this article] (2026). Euclid: Early Release Observations - A combined strong and weak lensing solution for Abell 2390 beyond its virial radius. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 706. doi: 10.1051/0004-6361/202556372. Full text in Research Archive Show summary

  Euclid is currently mapping the distribution of matter in the Universe in detail via the weak lensing signature of billions of distant galaxies. The weak lensing signal is most prominent around galaxy clusters and can extend up to distances well beyond their virial radius, thus constraining their total mass. Near the centre of clusters, where contamination by member galaxies is an issue, the weak lensing data can be complemented with strong lensing data. Strong lensing information can also diminish the uncertainty due to the mass-sheet degeneracy and provide high-resolution information about the distribution of matter in the centre of clusters. Here we present a joint strong and weak lensing analysis of the Euclid Early Release Observations of the cluster Abell 2390 at z = 0.228. Thanks to Euclid’s wide field of view of 0.5 deg^2, combined with its angular resolution in the visible band of 0.​​″13 and sampling of 0.​​″1 per pixel, we constrained the density profile in a wide range of radii, 30 kpc < r< 2000 kpc, from the inner region near the brightest cluster galaxy to beyond the virial radius of the cluster. We find relatively good consistency with earlier X-ray results based on assumptions of hydrostatic equilibrium and thus indirectly confirm the nearly relaxed state of this cluster. We also find consistency with previous results based on weak lensing data and ground-based observations of this cluster. From the combined strong + weak lensing profile, we derive the values of the viral mass M200 = (1.48 ± 0.29) × 10^15 M⊙, and virial radius r200 = (2.05 ± 0.13 Mpc), with error bars representing one standard deviation. The profile is well described by a Navarro-Frenk-White model with a concentration c = 6.5 and a small-scale radius of 230 kpc in the 30 kpc < r< 2000 kpc range that is best constrained by strong lensing and weak lensing data. Abell 2390 is the first of many examples where Euclid data will play a crucial role in providing masses for clusters. The large coverage provided by Euclid, combined with the depth of the observations and high angular resolution, will allow us to produce similar results in hundreds of other clusters with rich strong lensing data that is already available.

• Kubik, B.; Barbier, R.; Clemens, J.; Ferriol, S.; Secroun, A. & Smadja, G. [Show all 343 contributors for this article] (2026). Euclid preparation: LXXX. Overview of Euclid infrared detector performance from ground tests. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 707. doi: 10.1051/0004-6361/202555777. Full text in Research Archive Show summary

  This paper describes the objectives, design, and findings of the pre-launch ground characterisation campaigns of the Euclid infrared detectors. The aim of the ground characterisations is to evaluate the performance of the detectors, to calibrate the pixel response, and to derive the pixel response correction methods. The detectors have been tested and characterised in the facilities set up for this purpose. The pixel properties, including baseline, bad pixels, quantum efficiency, inter pixel capacitance, quantum efficiency, dark current, readout noise, conversion gain, response non-linearity, and image persistence were measured and characterised for each pixel. We describe in detail the test flow definition that allows us to derive the pixel properties and we present the data acquisition and data quality check software implemented for this purpose. We also outline the measurement protocols of all the pixel properties presented and we provide a comprehensive overview of the performance of the Euclid infrared detectors as derived after tuning the operating parameters of the detectors. The main conclusion of this work is that the performance of the infrared detectors Euclid meets the requirements. Pixels classified as non-functioning accounted for less than 0.2% of all science pixels. The interpixel capacitance (IPC) coupling is minimal, the cross-talk between adjacent pixels is less than 1% between adjacent pixels, and 95% of the pixels show a quantum efficienty (QE) greater than 80% across the entire spectral range of the Euclid mission. The conversion gain is approximately 0.52 ADU/e−, with a variation of less than 1% between channels of the same detector. The reset noise is approximately equal to 23 ADU rms after reference pixel correction. The readout noise of a single frame is approximately 13 e− rms while the signal estimator noise is measured at 7 e− rms in photometric mode and 9 e− rms in spectroscopic acquisition mode. The deviation from linear response at signal levels up to 80 ke− is less than 5% for 95% of the pixels. Median persistence amplitudes are less than 0.3% of the signal, though persistence exhibits significant spatial variation and differences between detectors.

• Guidi, M.; Veropalumbo, A.; Pugno, A.; Moresco, M.; Sefusatti, E. & Porciani, C. [Show all 311 contributors for this article] (2026). Euclid preparation: LXXVIII. Full-shape modelling of two-point and three-point correlation functions in real space. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 707. doi: 10.1051/0004-6361/202556177. Full text in Research Archive Show summary

  We investigated the accuracy and range of validity of the perturbative model for the two-point (2PCF) and three-point (3PCF) correlation functions in real space in view of the forthcoming analysis of the Euclid mission spectroscopic sample. We took advantage of clustering measurements from four snapshots of the Flagship I N-body simulations at z = {0.9,1.2,1.5,1.8}, which mimic the expected galaxy population in the ideal case, i.e. in the absence of observational effects such as purity and completeness. For the 3PCF we considered all available triangular configurations given a minimal separation (r_min). We first assessed the model performance by fixing the cosmological parameters and evaluating the goodness of fit provided by the perturbative bias expansion in the joint analysis of the two statistics, finding an overall agreement with the data down to separations of 20 h^−1 Mpc. Subsequently, we built on the state-of-the-art analysis and extended it to include the dependence on three cosmological parameters: the amplitude of scalar perturbations (As), the matter density (ωcdm), and the Hubble parameter (h). To achieve this goal, we developed an emulator capable of generating fast and robust modelling predictions for the two summary statistics, which thus enables an efficient sampling of the joint likelihood function. We therefore present the first joint full-shape analysis of the real-space 2PCF and 3PCF, testing the consistency and constraining power of the perturbative model across both probes and assessing its performance in a combined likelihood framework. We explored possible systematic uncertainties induced by the perturbative model at small scales, finding an optimal scale cut of r_min = 30 h^−1 Mpc for the 3PCF when imposing an additional limitation on the nearly isosceles triangular configurations included in the data vector. This work is part of a series of papers in which we validate theoretical models for galaxy clustering measurements in preparation for the Euclid mission.

• Urbano, M.; Duc, P. A.; Poulain, M.; Nucita, A. A.; Venhola, A. & Marchal, O. [Show all 315 contributors for this article] (2026). Euclid preparation: LXXIX. Using mock low surface brightness dwarf galaxies to probe Euclid Wide Survey detection capabilities. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 707. doi: 10.1051/0004-6361/202557270. Full text in Research Archive Show summary

  Local Universe dwarf galaxies can serve as both cosmological and mass assembly probes. Deep surveys have enabled the study of these objects down to the low surface brightness (LSB) regime. In this paper, we estimate Euclid’s dwarf detection capabilities as well as limits of its MERge processing function (MER pipeline), which is responsible for producing the stacked mosaics and final catalogues. To do this, we injected mock dwarf galaxies in a real Euclid Wide Survey (EWS) field in the VIS band and compared the input catalogue to the final MER catalogue. The mock dwarf galaxies were generated using simple Sérsic models with structural parameters (including size and surface brightness) drawn from observed dwarf galaxy catalogues. These simulations represent an idealised case in the sense they do not account for additional factors such as ellipticity, morphology, or crowding. To characterise the detected dwarfs, we used the mean surface brightness inside the effective radius SBe (in mag arcsec−2). The final MER catalogues achieve a completenesses of 91% for SBe ∈ [21, 24] and 54% for SBe ∈ [24, 28]. These numbers do not take into account possible contaminants, including confusion with background galaxies at the location of the dwarfs. After taking those effects into account, they respectively became 86% and 38%. The MER pipeline performs a final local background subtraction with a small mesh size, leading to a flux loss for galaxies with Re > 10″. By using the final MER mosaics and reinjecting this local background, we obtained an image in which we recover reliable photometric properties for objects under the arcminute scale. This background-reinjected product is thus suitable for the study of Local Universe dwarf galaxies. Euclid’s data reduction pipeline serves as a test bed for other deep surveys, particularly regarding background subtraction methods, a key issue in LSB science.

• Girardi, G.; Grazian, A.; Rodighiero, G.; Bisigello, L.; Gandolfi, G. & Bañados, E. [Show all 178 contributors for this article] (2026). Euclid: The first statistical census of dusty and massive objects in the ERO/Perseus field. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 706. doi: 10.1051/0004-6361/202556328. Full text in Research Archive Show summary

  Our comprehension of the history of star formation at z > 3 strongly relies on rest-frame ultraviolet observations. However, this selection systematically misses the dustiest and most massive sources, resulting in an incomplete census at earlier times. Infrared facilities such as Spitzer and the James Webb Space Telescope have shed light on a hidden population lying at z = 3 − 6 characterised by extreme red colours named HIEROs (HST-to-IRAC extremely red objects), identified by the colour criterion HE − ch2 > 2.25. Recently, Euclid Early Release Observations (EROs) have opened the possibility to further study such objects, exploiting the comparison between Euclid and ancillary Spitzer/IRAC observations. The aim of this study was to investigate the effectiveness of this synergy in characterising the population of a small test area of 232 arcmin^2. We utilised catalogues in the Perseus field across the VIS and NISP bands, supplemented by data from the four Spitzer channels and several ground-based MegaCam bands (u, g, r, Hα, i, and z) already included in the ERO catalogue. We selected 121 HIEROs by applying the H_E − ch2 > 2.25 colour cut, cleaned this sample of globular clusters and brown dwarfs, and then inspected by eye the multi-band cutouts of each source, ending with 42 reliable HIEROs. Photometric redshifts and other physical properties of the final sample were estimated using the spectral-energy-distribution-fitting software Bagpipes. From the zphot and M* values, we computed the galaxy stellar mass function at 3.5 < z < 5.5. When we exclude all galaxies that could host an active galactic nucleus, or whose stellar masses might be overestimated, we still find that the high-mass end of the galaxy stellar mass function is similar to previous estimates, indicating that the true value could be even higher. This investigation highlights the importance of a deeper study of this still mysterious population, in particular to assess its contribution to the cosmic star-formation rate density and its agreement with current galaxy evolution and formation models. These early results demonstrate Euclid’s capabilities to push the boundaries of our understanding of obscured star formation across a wide range of epochs.

• Larsen, S. S.; Ferguson, A. M.N.; Howell, J. M.; Annibali, F.; Cuillandre, J. C. & Hunt, L. K. [Show all 159 contributors for this article] (2025). Euclid : Star clusters in IC 342, NGC 2403, and Holmberg II. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 703. doi: 10.1051/0004-6361/202554661. Full text in Research Archive Show summary

  We examine the star cluster populations in the three nearby (3.20–3.45 Mpc) galaxies IC 342, NGC 2403, and Holmberg II, observed as part of the Euclid Early Release Observations programme. Our main focus in this paper is old globular clusters (GCs), for which the wide field of view and excellent image quality of Euclid offer substantial advantages over previous works. With respect to IC 342, in particular, this is the first study of stellar clusters that goes beyond its nuclear cluster. After a selection process based on size and magnitude criteria, followed by visual inspection, we identified 111 old (≳1 Gyr) GC candidates in IC 342, 49 in NGC 2403 (of which 15 were previously known), along with 7 in Holmberg II. In addition, a number of younger and/or intermediate-age candidates were identified. The colour distributions of GC candidates in the two larger galaxies show hints of bimodality, with peaks at (IE−{HE)0 = 0.36 and 0.79 in both IC 342 and NGC 2403, corresponding to metallicities of [Fe/H] ≈ −1.5 and [Fe/H] ≈ −0.5. These results are similar to those reported for the metal-poor and metal-rich GC subpopulations in the Milky Way. The luminosity functions of our GC candidates exhibit an excess of relatively faint objects, relative to a canonical, approximately Gaussian GC luminosity function (GCLF). Although some contamination from background galaxies and younger clusters might be present in our GC samples, we argue that the excess could be at least partially real. In particular, we find this could be the case for IC 342, where the excess objects could be similar to those previously identified, for example, in M101 and NGC 6946. The specific frequency of old GCs in IC 342, as determined on the basis of the brighter half of the GCLF, appears to be unusually low with SN ≈ 0.2–0.3. The combined luminosity function of young and intermediate-age clusters in all three galaxies is consistent with a power-law distribution of dN/dL ∝ L−2.3 ± 0.1. The total numbers of young clusters brighter than M(IE) = −8 in NGC 2403 and Holmberg II are comparable with those found in their Local Group counterparts, namely, M33 and the Small Magellanic Cloud, respectively.

• Bañados, E.; Brun, V. Le; Belladitta, S.; Momcheva, I.; Stern, D. & Wolf, J. [Show all 229 contributors for this article] (2025). Euclid: the potential of slitless infrared spectroscopy: a z = 5.4 quasar and new ultracool dwarfs. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 542(2), p. 1088–1102. doi: 10.1093/mnras/staf1274. Full text in Research Archive
• George, K.; Boselli, A.; Cuillandre, J. C.; Kümmel, M.; Lançon, A. & Bellhouse, C. [Show all 180 contributors for this article] (2025). Euclid: Early Release Observations of ram-pressure stripping in the Perseus cluster: Detection of parsec-scale star formation within the low surface brightness stripped tails of UGC 2665 and MCG +07-07-070. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 701. doi: 10.1051/0004-6361/202554836. Full text in Research Archive Show summary

  Euclid is delivering optical and near-infrared imaging data over 14 000 deg2 on the sky at spatial resolution and surface brightness levels that can be used to understand the morphological transformation of galaxies within groups and clusters. Using the Early Release Observations (ERO) of the Perseus cluster, we demonstrate the capability offered by Euclid in studying the nature of perturbations for galaxies in clusters. Filamentary structures are observed along the discs of two spiral galaxies, UGC 2665 and MCG +07-07-070, with no extended diffuse emission expected from tidal interactions at surface brightness levels of ∼30 mag arcsec−2. The detected features exhibit a good correspondence in morphology between optical and near-infrared wavelengths, with a surface brightness of ∼25 mag arcsec−2, and the knots within the features have sizes of ∼ 100 pc, as observed through IE imaging. Using the Euclid, CFHT, UVIT, and LOFAR 144 MHz radio continuum observations, we conducted a detailed analysis to understand the origin of the detected features. We constructed the Euclid IE−YE, YE−HE, and CFHT u − r, g − i colour-colour plane and show that these features contain recent star formation events, which are also indicated by their Hα and NUV emissions. Euclid colours alone are insufficient for studying stellar population ages in unresolved star-forming regions, which require multi-wavelength optical imaging data. There are features with red colours that can be explained by dust being stripped along with the gas in these regions. The morphological shape, orientation, and mean age of the stellar population, combined with the presence of extended radio continuum cometary tails can be consistently explained if these features formed during a recent ram-pressure stripping event. This result further confirms the exceptional qualities of Euclid in the study of galaxy evolution in dense environments.E

• Humphrey, A.; Cunha, P. A.C.; Bisigello, L.; Tortora, C.; Bolzonella, M. & Pozzetti, L. [Show all 234 contributors for this article] (2025). Euclid preparation: LXXV. Estimating galaxy physical properties using CatBoost chained regressors with attention. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 702. doi: 10.1051/0004-6361/202452468. Full text in Research Archive Show summary

  The Euclid Space Telescope will image about 14 000 deg2 of the extragalactic sky at visible and near-infrared wavelengths, providing a dataset of unprecedented size and richness that will facilitate a multitude of studies into the evolution of galaxies. Although spectroscopy will also be available for some of the galaxies, in the vast majority of cases the main source of information will come from broadband images and data products thereof (i.e. photometry). Therefore, there is a pressing need to identify or develop scalable yet reliable methodologies to estimate the redshift and physical properties of galaxies using broadband photometry from Euclid. Optionally, such methods could also include ground-based optical photometry. To address this need, we present a novel method developed as part of a ‘data challenge’ within the Euclid Collaboration to estimate the redshift, stellar mass, star-formation rate, specific star-formation rate, E(B − V), and age of galaxies using mock Euclid and ground-based photometry. The main novelty of our property-estimation pipeline is its use of the CatBoost implementation of gradient-boosted regression-trees together with chained regression and an intelligent, automatic optimisation of the training data. The pipeline also includes a computationally efficient method to estimate prediction uncertainties, and, in the absence of ground-truth labels, it provides accurate predictions for metrics of model performance up to z ~ 2. We applied our pipeline to several datasets consisting of mock Euclid broadband photometry and mock ground-based ugriz photometry, with the objective of evaluating the performance of our methodology for estimating the redshift and physical properties of galaxies detected in the Euclid Wide Survey. The statistical metrics of prediction residuals vary depending on which mock catalogue and filters are tested. Nonetheless, the quality of our photometric redshift and physical property estimates are highly competitive overall, validating our modelling approach. However, at z ≳ 3.5, the relative sparsity of galaxies resulted in unreliable redshift and physical property estimates, which we argue could be mitigated by building catalogues with better sampling of z ≳ 3.5 galaxies or by switching to the use of spectral energy distribution fitting in this regime. We also find that the inclusion of ground-based optical photometry significantly improves the quality of the property estimation, highlighting the importance of combining Euclid data with ancillary ground-based data from such surveys as the Vera C. Rubin Observatory Legacy Survey of Space and Time and UNIONS.

• Bergamini, P.; Meneghetti, M.; Angora, G.; Bazzanini, L.; Rosati, P. & Grillo, C. [Show all 265 contributors for this article] (2025). Euclid preparation: LXXIV. Euclidised observations of Hubble Frontier Fields and CLASH galaxy clusters. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 702. doi: 10.1051/0004-6361/202553984. Full text in Research Archive Show summary

  We present HST2EUCLID, a novel Python code to generate Euclid realistic mock images in the HE, JE, YE, and IE photometric bands based on panchromatic Hubble Space Telescope observations. The software was used to create a simulated database of Euclid images for the 27 galaxy clusters observed during the Cluster Lensing And Supernova survey with Hubble (CLASH) and the Hubble Frontier Fields (HFF) program. Since the mock images were generated from real observations, they incorporate, by construction, all the complexity of the observed galaxy clusters. The simulated Euclid data of the galaxy cluster MACS J0416.1−2403 were then used to explore the possibility of developing strong lensing models based on the Euclid data. In this context, complementary photometric or spectroscopic follow-up campaigns are required to measure the redshifts of multiple images and cluster member galaxies. By Euclidising six parallel blank fields obtained during the HFF program, we provide an estimate of the number of galaxies detectable in Euclid images per deg2 per magnitude bin (number counts) and the distribution of the galaxy sizes. Finally, we present a preview of the Chandra Deep Field South that will be observed during the Euclid Deep Survey and two examples of galaxy-scale strong lensing systems residing in regions of the sky covered by the Euclid Wide Survey. The methodology developed in this work lends itself to several additional applications, as simulated Euclid fields based on HST (or JWST) imaging with extensive spectroscopic information can be used to validate the feasibility of legacy science cases or to train deep learning techniques in advance, thus preparing for a timely exploitation of the Euclid Survey data.

• Torre, S. De La; Marulli, F.; Keihänen, Elina; Viitanen, A.; Viel, M. & Veropalumbo, A. [Show all 299 contributors for this article] (2025). Euclid preparation: LXXII. Three-dimensional galaxy clustering in configuration space: Two-point correlation function estimation. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 700. doi: 10.1051/0004-6361/202553927. Full text in Research Archive Show summary

  The two-point correlation function of the galaxy spatial distribution is a major cosmological observable that enables constraints on the dynamics and geometry of the Universe. The Euclid mission is aimed at performing an extensive spectroscopic survey of approximately 20–30 million Hα-emitting galaxies up to a redshift of about 2. This ambitious project seeks to elucidate the nature of dark energy by mapping the three-dimensional clustering of galaxies over a significant portion of the sky. This paper presents the methodology and software developed for estimating the three-dimensional two-point correlation function within the Euclid Science Ground Segment. The software is designed to overcome the significant challenges posed by the large and complex Euclid dataset, which involves millions of galaxies. The key challenges include efficient pair counting, managing computational resources, and ensuring the accuracy of the correlation function estimation. The software leverages advanced algorithms, including k-d tree, octree, and linked-list data partitioning strategies, to optimise the pair-counting process. These methods are crucial for handling the massive volume of data efficiently. The implementation also includes parallel processing capabilities using shared-memory open multi-processing to further enhance performance and reduce computation times. Extensive validation and performance testing of the software are presented. Those have been performed by using various mock galaxy catalogues to ensure that it meets the stringent accuracy requirement of the Euclid mission. The results indicate that the software is robust and can reliably estimate the two-point correlation function, which is essential for deriving cosmological parameters with high precision. Furthermore, the paper discusses the expected performance of the software during different stages of Euclid Wide Survey observations and forecasts how the precision of the correlation function measurements will improve over the mission’s timeline, highlighting the software’s capability to handle large datasets efficiently.

• Urbano, M.; Duc, P.-A.; Saifollahi, T.; Sola, E.; Lançon, A. & Voggel, K. [Show all 163 contributors for this article] (2025). Euclid: Early Release Observations of diffuse stellar structures and globular clusters as probes of the mass assembly of galaxies in the Dorado group. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 700. doi: 10.1051/0004-6361/202453534. Full text in Research Archive Show summary

  Deep surveys have helped to unveil the history of past and present galaxy mergers, and, in particular, uncovering their tidal debris and co-located globular clusters (GCs). Euclid ’s unique combination of capabilities (spatial resolution, depth, and wide sky coverage) will make it a groundbreaking tool for galactic archaeology in the Local Universe, bringing low-surface-brightness (LSB) science into the era of large-scale astronomical surveys. Euclid ’s Early Release Observations (ERO) demonstrate this potential with a field of view that includes several galaxies in the Dorado group. In this paper, we aim to derive from this image a mass assembly scenario for its main galaxies: NGC 1549, NGC 1553, and NGC 1546. We detected their internal and external diffuse structures, and identified candidate GCs. By analysing the colours and distributions of the diffuse structures and candidate GCs, we can place constraints on the galaxies’ mass assembly and merger histories. The results demonstrate that feature morphology, surface brightness, colours, and GC density profiles are consistent with galaxies that have undergone different merger scenarios. We classify NGC 1549 as a pure elliptical galaxy that has undergone a major merger. NGC 1553 appears to have recently transitioned from a late-type galaxy to early type, after a series of radial minor to intermediate mergers. NGC 1546 is a rare specimen of galaxy with an undisturbed disk and a prominent diffuse stellar halo, which we infer has been fed by minor mergers and then disturbed by the tidal effect from NGC 1553. Finally, we identify limitations specific to the observing conditions of this ERO, in particular, stray light in the visible and persistence in the near-infrared bands. Once these issues are addressed and the extended emission from LSB objects is preserved by the data-processing pipeline, the Euclid Wide Survey will allow for studies of the Local Universe to be extended to statistical ensembles over a large part of the extragalactic sky.

• Monaco, P.; Parimbelli, G.; Elkhashab, M. Y.; Salvalaggio, J.; Castro, T. & Lepinzan, M. D. [Show all 305 contributors for this article] (2025). Euclid preparation: LXXVI. Simulating thousands of Euclid spectroscopic skies. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 704. doi: 10.1051/0004-6361/202556459. Full text in Research Archive Show summary

  We present two extensive sets of 3500+1000 simulations of dark matter haloes on the past light cone and two corresponding sets of simulated (mock) galaxy catalogues that represent the spectroscopic sample of Euclid. The simulations were produced with the latest version of the code Pinocchio and provide the largest public set of simulated skies. The mock galaxy catalogues were obtained by populating haloes with galaxies using an halo occupation distribution (HOD) model extracted from the Flagship galaxy catalogue provided by Euclid Collaboration. The Geppetto set of 3500 simulated skies was obtained by tiling a 1.2 h−1 Gpc box to cover a light cone whose sky footprint is a circle with a radius of 30° for an area of 2763 deg2 and a minimum halo mass of 1.5 × 1011h−1 M⊙. The relatively small size of the box means that this set is unsuitable for measuring very large scales. The EuclidLargeBox set consists of 1000 simulations of 3.38 h−1 Gpc and has the same mass resolution and a footprint that covers half of the sky. It excludes the Milky Way zone of avoidance. From this, we produced a set of 1000 EuclidLargeMocks on the 30° radius footprint, whose comoving volume is fully contained in the simulation box. We validated the two sets of catalogues by analysing number densities, power spectra, and two-point correlation functions to show that the Flagship spectroscopic catalogue is consistent with being one of the realisations of the simulated sets. We noted small deviations, however, that are limited to the quadrupole at k > 0.2 h Mpc−1. We infer the cosmological parameters from these catalogues and demonstrate that using one realisation of EuclidLargeMocks in place of the Flagship mock produces the same posteriors to within the expected shift given by the sample variance. These simulated skies will be used for the galaxy clustering analysis of the Euclid Data Release 1 (DR1), and an even larger set of simulations is planned for the next releases.

• Nagam, B. C.; Barroso, J. A. Acevedo; Wilde, J.; Andika, I. T.; Manjón-García, A. & Pearce-Casey, R. [Show all 195 contributors for this article] (2025). Euclid: Finding strong gravitational lenses in the early release observations using convolutional neural networks. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 702. doi: 10.1051/0004-6361/202554132. Full text in Research Archive Show summary

  Several new galaxy-galaxy strong gravitational lenses have been detected in the early release observations (ERO) from Euclid. The all-sky survey is expected to find 170 000 new systems, which are expected to greatly enhancing studies of dark matter and dark energy, and to constrain the cosmological parameters better. As a first step, we visually inspect all galaxies in one of the ERO fields (Perseus) to identify candidate strong-lensing systems and compared them to the predictions from convolutional neural networks (CNNs). The entire ERO dataset is too large for an expert visual inspection, however. In this paper, we therefore extend the CNN analysis to the whole ERO dataset and use different CNN architectures and methods. Using five CNN architectures, we identified 8469 strong gravitational lens candidates from IE-band cutouts of 13 Euclid ERO fields and narrowed them down to 97 through visual inspection. The sample includes 14 grade A and 31 grade B candidates. We present the spectroscopic confirmation of a strong gravitational lensing candidate, EUCL J081705.61+702348.8. The foreground lensing galaxy, an early-type system at z = 0.335, and the background source, a star-forming galaxy at z = 1.475 with [O II] emission, are both identified. The lens modelling with the Euclid strong lens modelling pipeline revealed two distinct arcs in a lensing configuration, with an Einstein radius of 1.″18 ± 0.″03. This confirms the lensing nature of the system. These findings demonstrate that CNN-based candidate selection followed by visual inspection provides an effective approach for identifying strong lenses in Euclid data. They also highlight areas for improvement in future large-scale implementations.

• Koyama, K.; Pamuk, S.; Casas, S.; Bose, B.; Carrilho, P. & Sáez-Casares, I. [Show all 277 contributors for this article] (2025). Euclid preparation: LXXI. Simulations and nonlinearities beyond ΛCDM. 3. Constraints on f(R) models from the photometric primary probes. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 698. doi: 10.1051/0004-6361/202452184. Full text in Research Archive Show summary

  We study the constraint on f ( R ) gravity that can be obtained by photometric primary probes of the Euclid mission. Our focus is the dependence of the constraint on the theoretical modelling of the nonlinear matter power spectrum. In the Hu–Sawicki f ( R ) gravity model, we consider four different predictions for the ratio between the power spectrum in f ( R ) and that in Λ cold dark matter (ΛCDM): a fitting formula, the halo model reaction approach, ReACT , and two emulators based on dark matter only N -body simulations, FORGE and e-Mantis . These predictions are added to the MontePython implementation to predict the angular power spectra for weak lensing (WL), photometric galaxy clustering, and their cross-correlation. By running Markov chain Monte Carlo, we compare constraints on parameters and investigate the bias of the recovered f ( R ) parameter if the data are created by a different model. For the pessimistic setting of WL, one-dimensional bias for the f ( R ) parameter, log 10 | f R 0 |, is found to be 0.5 σ when FORGE is used to create the synthetic data with log 10 | f R 0 | = −5.301 and fitted by e-Mantis . The impact of baryonic physics on WL is studied by using a baryonification emulator, BCemu . For the optimistic setting, the f ( R ) parameter and two main baryonic parameters are well constrained despite the degeneracies among these parameters. However, the difference in the nonlinear dark matter prediction can be compensated for the adjustment of baryonic parameters, and the one-dimensional marginalised constraint on log 10 | f R 0 | is biased. This bias can be avoided in the pessimistic setting at the expense of weaker constraints. For the pessimistic setting, using the ΛCDM synthetic data for WL, we obtain the prior-independent upper limit of log 10 | f R 0 | < −5.6. Finally, we implement a method to include theoretical errors to avoid the bias due to inaccuracies in the nonlinear matter power spectrum prediction.

• Rácz, G.; Breton, M.-A.; Fiorini, B.; Brun, A. M. C. Le; Winther, Hans Arnold & Sakr, Z. [Show all 277 contributors for this article] (2025). Euclid preparation: LXIII. Simulations and non-linearities beyond Lambda cold dark matter. 2. Results from non-standard simulations. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 695. doi: 10.1051/0004-6361/202452185. Full text in Research Archive Show summary

  The Euclid mission will measure cosmological parameters with unprecedented precision. To distinguish between cosmological models, it is essential to generate realistic mock observables from cosmological simulations that were run in both the standard Λ-cold-dark-matter (ΛCDM) paradigm and in many non-standard models beyond ΛCDM. We present the scientific results from a suite of cosmological N -body simulations using non-standard models including dynamical dark energy, k -essence, interacting dark energy, modified gravity, massive neutrinos, and primordial non-Gaussianities. We investigate how these models affect the large-scale-structure formation and evolution in addition to providing synthetic observables that can be used to test and constrain these models with Euclid data. We developed a custom pipeline based on the Rockstar halo finder and the nbodykit large-scale structure toolkit to analyse the particle output of non-standard simulations and generate mock observables such as halo and void catalogues, mass density fields, and power spectra in a consistent way. We compare these observables with those from the standard ΛCDM model and quantify the deviations. We find that non-standard cosmological models can leave large imprints on the synthetic observables that we have generated. Our results demonstrate that non-standard cosmological N -body simulations provide valuable insights into the physics of dark energy and dark matter, which is essential to maximising the scientific return of Euclid .

• Saifollahi, T.; Lançon, A.; Cantiello, M.; Cuillandre, J. C.; Bethermin, M. & Carollo, D. [Show all 187 contributors for this article] (2025). Euclid: Early Release Observations – Interplay between dwarf galaxies and their globular clusters in the Perseus galaxy cluster. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 703. doi: 10.1051/0004-6361/202554667. Full text in Research Archive Show summary

  We present an analysis of globular clusters (GCs) of dwarf galaxies in the Perseus galaxy cluster that explores the relationship between dwarf galaxy properties and their GCs. Our focus is on GC numbers (NGC) and GC half-number radii (RGC) around dwarf galaxies, and their relations with host galaxy stellar masses (M*), central surface brightnesses (μ0), and effective radii (Re). This work is unique due to its large sample size and the absence of pre-selection based on μ0 and RGC for dwarf galaxies. Interestingly, we find that at a given stellar mass, RGC is almost independent of the host galaxy μ0 and Re, while RGC/Re depends on μ0 and Re. Lower surface brightness and diffuse dwarf galaxies show RGC/Re ≈ 1, while higher surface brightness and compact dwarf galaxies show RGC/Re ≈ 1.5–2. This means that for dwarf galaxies of similar stellar mass, the GCs have a similar median extent; however, their distribution is different from the field stars of their host. Additionally, low surface brightness and diffuse dwarf galaxies on average have a higher NGC than high surface brightness and compact dwarf galaxies at any given stellar mass. We also find that ultra-diffuse galaxies (UDGs) and non-UDGs in the sample have a similar RGC, while UDGs have a smaller RGC/Re (typically less than one) and a three to four times higher NGC than non-UDGs. Furthermore, when examining nucleated versus non-nucleated dwarf galaxies, we found that for M* > 108 M⊙, nucleated dwarf galaxies seem to have a smaller RGC and RGC/Re, with no significant differences seen between their NGC except at M* < 108 M⊙, where the nucleated dwarf galaxies tend to have a higher NGC. Lastly, we explored the stellar-to-halo mass ratio (SHMR) of dwarf galaxies (halo mass based on NGC) and conclude that the Perseus cluster dwarf galaxies follow the expected SHMR at z = 0 extrapolated down to M* = 106 M⊙.

• Lepori, F.; Schulz, S.; Tutusaus, I.; Breton, M.-A.; Saga, S. & Viglione, C. [Show all 133 contributors for this article] (2025). Euclid: Relativistic effects in the dipole of the two-point correlation function. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 694. doi: 10.1051/0004-6361/202452531. Full text in Research Archive Show summary

  Gravitational redshift and Doppler effects give rise to an antisymmetric component of the galaxy correlation function when cross-correlating two galaxy populations or two different tracers. In this paper, we assess the detectability of these effects in the Euclid spectroscopic galaxy survey. We model the impact of gravitational redshift on the observed redshift of galaxies in the Flagship mock catalogue using a Navarro–Frenk–White profile for the host haloes. We isolate these relativistic effects, largely subdominant in the standard analysis, by splitting the galaxy catalogue into two populations of faint and bright objects and estimating the dipole of their cross-correlation in four redshift bins. In the simulated catalogue, we detect the dipole signal on scales below 30  h −1  Mpc, with detection significances of 4 σ and 3 σ in the two lowest redshift bins, respectively. At higher redshifts, the detection significance drops below 2 σ . Overall, we estimate the total detection significance in the Euclid spectroscopic sample to be approximately 6 σ . We find that on small scales, the major contribution to the signal comes from the nonlinear gravitational potential. Our study on the Flagship mock catalogue shows that this observable can be detected in Euclid Data Release 2 and beyond.

• Martín, E.L.; Žerjal, M.; Bouy, H.; Martin-Gonzalez, D.; Torres, Torres Muñoz & Barrado, D. [Show all 184 contributors for this article] (2025). Euclid: Early Release Observations - A glance at free-floating newborn planets in the σ Orionis cluster. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 697. doi: 10.1051/0004-6361/202450793. Full text in Research Archive Show summary

  We provide an early assessment of the imaging capabilities of the Euclid space mission to deeply probe nearby star-forming regions and associated very young open clusters, and in particular, to determine to which extent it can shed light into the newborn free- floating planet population. This paper focusses on a low-reddening region observed in just one Euclid pointing. The dust and gas has been cleared out from the region by the hot σ Orionis star. One late-M and six known spectroscopically confirmed L-type ultracool members in the σ Orionis cluster were used as benchmarks to provide a high-purity procedure to select new candidate members with Euclid. The exquisite angular resolution and depth delivered by the Euclid instruments allowed us to focus on bona fide point sources. A cleaned sample of σ Orionis cluster substellar members was produced, and the initial mass function (IMF) was estimated by combining Euclid and Gaia data. Our σ Orionis substellar IMF is consistent with a power-law distribution without a significant steepening at the planetary-mass end. No evidence of a low-mass cutoff is found down to the detection limit of this study at 4 Jupiter masses in the very young σ Orionis open cluster.

• Cuillandre, J.-C.; Bolzonella, M.; Boselli, A.; Marleau, F.R.; Mondelin, M. & Sorce, J.G. [Show all 202 contributors for this article] (2025). Euclid: Early Release Observations – Overview of the Perseus cluster and analysis of its luminosity and stellar mass functions. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 697. doi: 10.1051/0004-6361/202450808. Full text in Research Archive Show summary

  The Euclid Early Release Observations (ERO) programme targeted the Perseus cluster of galaxies, gathering deep data in the central region of the cluster over 0.7 deg2, including the cluster core up to 0.25 r200. The dataset reaches a point-source depth of IE = 28.0 (YE, JE, HE = 25.3), AB magnitudes at 5 σ with a 0′′.16 (0′′.48) full width at half maximum (FWHM), and a surface brightness limit of 30.1 (29.2) mag arcsec−2 for radially integrated galaxy profiles. The exceptional depth and spatial resolution of this wide-field multi-band data enable simultaneous detection and characterisation of both bright galaxies and low surface brightness ones, along with their globular cluster systems, from the optical to the near-infrared (NIR). Cluster membership was determined using several methods in order to maximise the completeness and minimise the contamination of foreground and background sources. We adopted a catalogue of 1100 dwarf galaxies, detailed in the corresponding ERO paper, that includes their photometric and structural properties. We identified all other sources in the Euclid images and obtained accurate photometric measurements using AutoProf or AstroPhot for 137 bright cluster galaxies and SourceExtractor for half a million compact sources. This study advances beyond previous analyses of the cluster and enables a range of scientific investigations, which are summarised here. We derived the luminosity and stellar mass functions (LF and SMF) of the Perseus cluster in the Euclid IE band thanks to supplementary u, g, r, i, z, and Hα data from the Canada-France-Hawai’i Telescope (CFHT). Our LF and SMF are the deepest recorded for the Perseus cluster, highlighting the groundbreaking capabilities of the Euclid telescope. We fit the LF and SMF with a Schechter plus Gaussian model. The LF features a dip at M(IE) ≃ −19 and a faint-end slope of αS ≃ −1.2 to −1.3. The SMF displays a low-mass-end slope of αS ≃ −1.2 to −1.35. These observed slopes are flatter than those predicted for dark matter halos in cosmological simulations, offering significant insights for models of galaxy formation and evolution.

• Saifollahi, T.; Voggel, K.; Lançon, A.; Cantiello, Michele; Raj, M.A. & Cuillandre, J.-C. [Show all 199 contributors for this article] (2025). Euclid: Early Release Observations – Globular clusters in the Fornax galaxy cluster, from dwarf galaxies to the intracluster field. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 697. doi: 10.1051/0004-6361/202450784. Full text in Research Archive Show summary

  We present an analysis of Euclid observations of a 0.6 deg2 field in the central region of the Fornax galaxy cluster that were acquired during the performance verification phase. With these data, we investigated the potential of Euclid to identify globular clusters (GCs) at 20 Mpc and validated the search methods using artificial GCs and known GCs within the field from the literature. Our analysis of artificial GCs injected into the data shows that Euclid’s data in the IE band is 80% complete at about IE ∼ 26.0 mag (MV ~ –5.0 mag), and it resolves GCs as small as rh = 2.5 pc. In the IE band, we detected more than 95% of the known GCs from previous spectroscopic surveys and GC candidates of the ACS Fornax Cluster Survey, of which more than 80% are resolved. We identify more than 5000 new GC candidates within the field of view down to IE = 25.0 mag, about 1.5 mag fainter than the typical GC luminosity function turn-over magnitude, and we investigated their spatial distribution within the intracluster field. We then focused on the GC candidates around dwarf galaxies and investigated their numbers, stacked luminosity distribution, and stacked radial distribution. While the overall GC properties are consistent with those in the literature, we found an interesting over-representation of relatively bright candidates within a small number of relatively GC-rich dwarf galaxies. Our work confirms the capabilities of Euclid data in detecting GCs and separating them from foreground and background contaminants at a distance of 20 Mpc, particularly for low GC-count systems such as dwarf galaxies.

• Mondelin, M.; Bournaud, F.; Cuillandre, J.-C.; Codis, S.; Stone, C. & Bolzonella, M. [Show all 113 contributors for this article] (2025). Euclid: Early Release Observations – The surface brightness and colour profiles of the far outskirts of galaxies in the Perseus cluster. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 699. doi: 10.1051/0004-6361/202554838. Full text in Research Archive Show summary

  The Perseus field captured by Euclid as part of its Early Release Observations provides a unique opportunity to study cluster environment ranging from outskirts to dense regions. Leveraging unprecedented optical and near-infrared depths, we investigate the stellar structure of massive disc galaxies in this field. This study focuses on outer disc profiles, including simple exponential (Type I), down-bending break (Type II) and up-bending break (Type III) profiles, and their associated colour gradients, to trace late assembly processes across various environments. Type II profiles, though relatively rare in high dense environments, appear stabilised by internal mechanisms like bars and resonances, even within dense cluster cores. Simulations suggest that in dense environments, Type II profiles tend to evolve into Type I profiles over time. Type III profiles often exhibit small colour gradients beyond the break, hinting at older stellar populations, potentially due to radial migration or accretion events. We analyse correlations between galaxy mass, morphology, and profile types. Mass distributions show weak trends of decreasing mass from the centre to the outskirts of the Perseus cluster. Type III profiles become more prevalent, while Type I profiles decrease in lower-mass galaxies with cluster centric distance. Type I profiles dominate in spiral galaxies, while Type III profiles are more common in S0 galaxies. Type II profiles are consistently observed across all morphological types. While the limited sample size restricts statistical power, our findings shed light on the mechanisms shaping galaxy profiles in cluster environments. Future work should extend observations to the cluster outskirts to enhance statistical significance and explore looser environments. Additionally, 3D velocity maps are needed to achieve a non-projected view of galaxy positions, offering deeper insights into spatial distribution and dynamics.

• Cuillandre, J.-C.; Bertin, E.; Bolzonella, M.; Bouy, H.; Gwyn, S. & Isani, S. [Show all 264 contributors for this article] (2025). Euclid: Early Release Observations – Programme overview and pipeline for compact- and diffuse-emission photometry. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 697. doi: 10.1051/0004-6361/202450803. Full text in Research Archive Show summary

  The Euclid Early Release Observations (ERO) showcase Euclid’s capabilities in advance of its main mission by targeting 17 astronomical objects, including galaxy clusters, nearby galaxies, globular clusters, and star-forming regions. A total of 24 hours of observing time was allocated in the early months of operation, and the scientific community was engaged through an early public data release. We describe the development of the ERO pipeline to create visually compelling images while simultaneously meeting the scientific demands within months of launch by leveraging a pragmatic data-driven development strategy. The pipeline’s key requirements are to preserve the image quality and to provide flux calibration and photometry for compact and extended sources. The pipeline’s five pillars are removal of instrumental signatures, astrometric calibration, photometric calibration, image stacking, and the production of science-ready catalogues for both the VIS and NISP instruments. We report a point spread function (PSF) with a full width at half maximum of 0.16 arcsec in the optical IE-band and 0.49 arcsec in the near-infrared (NIR) bands YE, JE, and HE. Our VIS mean absolute flux calibration is accurate to about 1%, and the accuracy is 10% for NISP due to a limited calibration set; both instruments have considerable colour terms for individual sources. The median depth is 25.3 and 23.2 AB mag with a signal-to-noise ratio (S/N) of ten for galaxies, while it is 27.1 and 24.5 AB mag at an S/N of five for point sources for VIS and NISP, respectively. Euclid’s ability to observe diffuse emission is exceptional due to its extended PSF nearly matching a pure diffraction halo, the best ever achieved by a wide-field high-resolution imaging telescope. Euclid offers unparalleled capabilities for exploring the low-surface brightness (LSB) Universe across all scales, providing high precision within a wide field of view (FoV), and opening a new observational window in the NIR. Median surface-brightness levels of 29.5 and 27.9, AB mag arcsec−2 are achieved for VIS and NISP, respectively, for detecting a 10 × 10 arcsec extended feature at the 1 σ level.

• Mellier, Y.; Abdurro’uf, .; Barroso, J.A. Acevedo; Achúcarro, A.; Adamek, J. & Adam, R. [Show all 1,138 contributors for this article] (2025). Euclid I. Overview of the Euclid mission. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 697. doi: 10.1051/0004-6361/202450810. Full text in Research Archive Show summary

  The current standard model of cosmology successfully describes a variety of measurements, but the nature of its main ingredients, dark matter and dark energy, remains unknown. Euclid is a medium-class mission in the Cosmic Vision 2015–2025 programme of the European Space Agency (ESA) that will provide high-resolution optical imaging, as well as near-infrared imaging and spectroscopy, over about 14 000 deg2 of extragalactic sky. In addition to accurate weak lensing and clustering measurements that probe structure formation over half of the age of the Universe, its primary probes for cosmology, these exquisite data will enable a wide range of science. This paper provides a high-level overview of the mission, summarising the survey characteristics, the various data-processing steps, and data products. We also highlight the main science objectives and expected performance.

• Barroso, J.A. Acevedo; O’Riordan, C.M.; Clément, B.; Tortora, C.; Collett, T.E. & Courbin, F. [Show all 209 contributors for this article] (2025). Euclid: The Early Release Observations Lens Search Experiment. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 697. doi: 10.1051/0004-6361/202451868. Full text in Research Archive Show summary

  We investigated the ability of the Euclid telescope to detect galaxy-scale gravitational lenses. To do so, we performed a systematic visual inspection of the 0.7 deg2 Euclid Early Release Observations data towards the Perseus cluster using both the high-resolution IE band and the lower-resolution YE , JE, and HE bands. Each extended source brighter than magnitude 23 in IE was inspected by 41 expert human classifiers. This amounts to 12086 stamps of 10″ × 10″. We found 3 grade A and 13 grade B candidates. We assessed the validity of these 16 candidates by modelling them and checking that they are consistent with a single source lensed by a plausible mass distribution. Five of the candidates pass this check, five others are rejected by the modelling, and six are inconclusive. Extrapolating from the five successfully modelled candidates, we infer that the full 14 000 deg2 of the Euclid Wide Survey should contain 100 000−30 000+ 70 000 galaxy-galaxy lenses that are both discoverable through visual inspection and have valid lens models. This is consistent with theoretical forecasts of 170 000 discoverable galaxy-galaxy lenses in Euclid. Our five modelled lenses have Einstein radii in the range 0'.'68 < θE < 1″.24, but their Einstein radius distribution is on the higher side when compared to theoretical forecasts. This suggests that our methodology is likely missing small-Einstein-radius systems. Whilst it is implausible to visually inspect the full Euclid dataset, our results corroborate the promise that Euclid will ultimately deliver a sample of around 105 galaxy-scale lenses.

• Marleau, F.R.; Cuillandre, J.-C.; Cantiello, M.; Carollo, D.; Duc, P.-A. & Habas, R. [Show all 196 contributors for this article] (2025). Euclid: Early Release Observations – Dwarf galaxies in the Perseus galaxy cluster. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 697. doi: 10.1051/0004-6361/202450799. Full text in Research Archive Show summary

  We make use of the unprecedented depth, spatial resolution, and field of view of the Euclid Early Release Observations (EROs) of the Perseus galaxy cluster to detect and characterise the dwarf galaxy population in this massive system. Using a dedicated annotation tool, the Euclid high-resolution VIS and combined VIS+Near Infrared Spectrometer and Photometer (NISP) colour images were visually inspected and dwarf galaxy candidates were identified. Their morphologies, the presence of nuclei, and their globular cluster (GC) richness were visually assessed, complementing an automatic detection of the GC candidates. Structural and photometric parameters, including Euclid filter colours, were extracted from two-dimensional fitting. Based on this analysis, a total of 1100 dwarf candidates were found across the image; 606 of these appear to be new identifications. The majority (96%) are classified as dwarf ellipticals, 53% are nucleated, 26% are GC-rich, and 6% show disturbed morphologies. A relatively high fraction of galaxies, 8%, are categorised as ultra diffuse galaxies. The majority of the dwarfs follow the expected scaling relations of galaxies. Globally, the GC specific frequency, SN , of the Perseus dwarf candidates is intermediate between those measured in the Virgo and Coma clusters. While the dwarf candidates with the largest GC counts are found throughout the Euclid field of view, the dwarfs located around the east–west strip, where most of the brightest cluster members are found, exhibit higher SN values on average. The spatial distribution of the dwarfs, GCs, and intracluster light show a main iso-density and isophotal centre displaced to the west of the bright galaxy light distribution. The ERO imaging of the Perseus cluster demonstrates the unique capability of Euclid to concurrently detect and characterise large samples of dwarf galaxies, their nuclei, and their GC systems, allowing us to construct a detailed picture of the formation and evolution of galaxies over a wide range of mass scales and environments.

• Bellhouse, C.; Golden-Marx, J.B.; Bamford, S.P.; Hatch, N.A.; Kluge, M. & Ellien, A. [Show all 294 contributors for this article] (2025). Euclid preparation: LXX. Forecasting detection limits for intracluster light in the Euclid Wide Survey. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 698. doi: 10.1051/0004-6361/202553887. Full text in Research Archive Show summary

  The intracluster light (ICL) permeating galaxy clusters is a tracer of the cluster assembly history and potentially a tracer of their dark matter structure. In this work, we explore the capability of the Euclid Wide Survey to detect ICL using HE-band mock images. We simulated clusters across a range of redshifts (0.3–1.8) and halo masses (1013.9–1015.0 M⊙) using an observationally motivated model of ICL. We identified a 50–200 kpc circular annulus around the brightest cluster galaxy (BCG) in which the signal-to-noise ratio of the ICL is maximised and used the S/N within this aperture as our figure of merit for ICL detection. We compared three state-of-the-art methods for ICL detection and found that a method that performs simple aperture photometry after high-surface brightness source masking is able to detect ICL with minimal bias for clusters more massive than 1014.2 M⊙. The S/N of the ICL detection is primarily limited by the redshift of the cluster, which is driven by cosmological dimming rather than the mass of the cluster. Assuming the ICL in each cluster contains 15% of the stellar light, we forecast that Euclid will be able to measure the presence of ICL in up to ∼80 000 clusters of >1014.2 M⊙ between z = 0.3 and 1.5 with an S/N>3. Half of these clusters will reside below z = 0.75, and the majority of those below z = 0.6 will be detected with an S/N>20. A few thousand clusters at 1.3<z<1.5 will have ICL detectable with an S/N >3. The surface brightness profile of the ICL model is strongly dependent on both the mass of the cluster and the redshift at which it is observed so that the outer ICL is best observed in the most massive clusters of >1014.7 M⊙. Euclid will detect the ICL at a distance of more than 500 kpc from the BCG, up to z = 0.7, in several hundred of these massive clusters over its large survey volume.

• Ragagnin, A.; Saro, A.; Andreon, S.; Biviano, A.; Dolag, K. & Ettori, S. [Show all 242 contributors for this article] (2025). Euclid preparation: LXVI. Impact of line-of-sight projections on the covariance between galaxy cluster multi-wavelength observable properties: Insights from hydrodynamic simulations. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 695. doi: 10.1051/0004-6361/202451347. Full text in Research Archive Show summary

  Context. Cluster cosmology can benefit from combining multi-wavelength studies. In turn, these studies benefit from a characterisation of the correlation coefficients among different mass-observable relations. Aims. In this work, we aim to provide information on the scatter, skewness, and covariance of various mass-observable relations in galaxy clusters in cosmological hydrodynamic simulations. This information will help future analyses improve the general approach to accretion histories and projection effects, as well as to model mass-observable relations for cosmology studies. Methods. We identified galaxy clusters in Magneticum Box2b simulations with masses of M200c > 1014 M⊙ at redshifts of z = 0.24 and z = 0.90. Our analysis included Euclid-derived properties such as richness, stellar mass, lensing mass, and concentration. Additionally, we investigated complementary multi-wavelength data, including X-ray luminosity, integrated Compton-y parameter, gas mass, and temperature. We then examined the impact of projection effects on mass-observable residuals and correlations. Results. We find that at intermediate redshift (z = 0.24), projection effects have the greatest impact of lensing concentration, richness, and gas mass in terms of the scatter and skewness of the log-residuals of scaling relations. The contribution of projection effects can be significant enough to boost a spurious hot- versus cold-baryon correlations and consequently hide underlying correlations due to halo accretion histories. At high redshift (z = 0.9), the richness has a much lower scatter (of log-residuals), while the quantity that is most impacted by projection effects is the lensing mass. The lensing concentration reconstruction, in particular, is affected by deviations of the reduced-shear profile shape from that derived using a Navarro-Frenk-White (NFW) profile; the amount of interlopers in the line of sight, on the other hand, is not as important.

• Böhringer, H.; Chon, G.; Cucciati, O.; Dannerbauer, H.; Bolzonella, M. & Lucia, G. De [Show all 265 contributors for this article] (2025). Euclid preparation: LV. Exploring the properties of proto-clusters in the Simulated Euclid Wide Survey. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 693. doi: 10.1051/0004-6361/202451683. Full text in Research Archive Show summary

  Galaxy proto-clusters are receiving increased interest since most of the processes shaping the structure of clusters of galaxies and their galaxy population happen at the early stages of their formation. The Euclid Survey will provide a unique opportunity to discover a large number of proto-clusters over a large fraction of the sky (14 500 deg2). In this paper, we explore the expected observational properties of proto-clusters in the Euclid Wide Survey by means of theoretical models and simulations. We provide an overview of the predicted proto-cluster extent, galaxy density profiles, mass-richness relations, abundance, and sky-filling as a function of redshift. Useful analytical approximations for the functions of these properties are provided. The focus is on the redshift range z = 1.5 − 4. In particular we discuss the density contrast with which proto-clusters can be observed against the background in the galaxy distribution if photometric galaxy redshifts are used as supplied by the ESA Euclid mission together with the ground-based photometric surveys. We show that the obtainable detection significance is sufficient to find large numbers of interesting proto-cluster candidates. For quantitative studies, additional spectroscopic follow-up is required to confirm the proto-clusters and establish their richness.

• Kovačić, I.; Baes, M.; Nersesian, A.; Andreadis, N.; Nemani, L. & Abdurro'uf [Show all 289 contributors for this article] (2025). Euclid preparation: LXVIII. Extracting physical parameters from galaxies with machine learning. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 695. doi: 10.1051/0004-6361/202453111. Full text in Research Archive Show summary

  The Euclid mission is generating a vast amount of imaging data in four broadband filters at a high angular resolution. This data will allow for the detailed study of mass, metallicity, and stellar populations across galaxies that will constrain their formation and evolutionary pathways. Transforming the Euclid imaging for large samples of galaxies into maps of physical parameters in an efficient and reliable manner is an outstanding challenge. Here, we investigate the power and reliability of machine learning techniques to extract the distribution of physical parameters within well-resolved galaxies. We focus on estimating stellar mass surface density, mass-averaged stellar metallicity, and age. We generated noise-free synthetic high-resolution (100 pc × 100 pc) imaging data in the Euclid photometric bands for a set of 1154 galaxies from the TNG50 cosmological simulation. The images were generated with the SKIRT radiative transfer code, taking into account the complex 3D distribution of stellar populations and interstellar dust attenuation. We used a machine learning framework to map the idealised mock observational data to the physical parameters on a pixel-by-pixel basis. We find that stellar mass surface density can be accurately recovered with a ≤0.130 dex scatter. Conversely, stellar metallicity and age estimates are, as expected, less robust, but they still contain significant information that originates from underlying correlations at a sub-kiloparsec scales between stellar mass surface density and stellar population properties. As a corollary, we show that TNG50 follows a spatially resolved mass-metallicity relation that is consistent with observations. Due to its relatively low computational and time requirements, which has a time-frame of minutes without dedicated high performance computing infrastructure once it has been trained, our method allows for fast and robust estimates of the stellar mass surface density distributions of nearby galaxies from four-filter Euclid imaging data. Equivalent estimates of stellar population properties (stellar metallicity and age) are less robust but still hold value as first-order approximations across large samples.

• Csizi, B.; Schrabback, T.; Grandis, S.; Hoekstra, H.; Jansen, H. & Linke, L. [Show all 261 contributors for this article] (2025). Euclid preparation: LXVII. Deep learning true galaxy morphologies for weak lensing shear bias calibration. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 695. doi: 10.1051/0004-6361/202452129. Full text in Research Archive Show summary

  To date, galaxy image simulations for weak lensing surveys usually approximate the light profiles of all galaxies as a single or double Sérsic profile, neglecting the influence of galaxy substructures and morphologies deviating from such a simplified parametric characterisation. While this approximation may be sufficient for previous data sets, the stringent cosmic shear calibration requirements and the high quality of the data in the upcoming Euclid survey demand a consideration of the effects that realistic galaxy substructures and irregular shapes have on shear measurement biases. Here we present a novel deep learning-based method to create such simulated galaxies directly from Hubble Space Telescope (HST) data. We first build and validate a convolutional neural network based on the wavelet scattering transform to learn noise-free representations independent of the point-spread function (PSF) of HST galaxy images. These can be injected into simulations of images from Euclid’s optical instrument VIS without introducing noise correlations during PSF convolution or shearing. Then, we demonstrate the generation of new galaxy images by sampling from the model randomly as well as conditionally. In the latter case, we fine-tune the interpolation between latent space vectors of sample galaxies to directly obtain new realistic objects following a specific Sérsic index and half-light radius distribution. Furthermore, we show that the distribution of galaxy structural and morphological parameters of our generative model matches the distribution of the input HST training data, proving the capability of the model to produce realistic shapes. Next, we quantify the cosmic shear bias from complex galaxy shapes in Euclid-like simulations by comparing the shear measurement biases between a sample of model objects and their best-fit double-Sérsic counterparts, thereby creating two separate branches that only differ in the complexity of their shapes. Using the Kaiser, Squires, and Broadhurst shape measurement algorithm, we find a multiplicative bias difference between these branches with realistic morphologies and parametric profiles on the order of (6.9 ± 0.6)×10−3 for a realistic magnitude-Sérsic index distribution. Moreover, we find clear detection bias differences between full image scenes simulated with parametric and realistic galaxies, leading to a bias difference of (4.0 ± 0.9)×10−3 independent of the shape measurement method. This makes complex morphology relevant for stage IV weak lensing surveys, exceeding the full error budget of the Euclid Wide Survey (Δμ1, 2 < 2 × 103).

• Ingoglia, L.; Sereno, M.; Farrens, S.; Giocoli, C.; Baumont, L. & Lesci, G.F. [Show all 281 contributors for this article] (2025). Euclid preparation: LXV. Determining the weak lensing mass accuracy and precision for galaxy clusters. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 695. doi: 10.1051/0004-6361/202452122. Full text in Research Archive Show summary

  The ability to measure unbiased weak-lensing (WL) masses is a key ingredient to exploit galaxy clusters as a competitive cosmological probe with the ESA Euclid survey or future missions. We investigate the level of accuracy and precision of cluster masses measured with the Euclid data processing pipeline. We use the DEMNUni-Cov N-body simulations to assess how well the WL mass probes the true halo mass, and, then, how well WL masses can be recovered in the presence of measurement uncertainties. We consider different halo mass density models, priors, and mass point estimates, that is the biweight, mean, and median of the marginalised posterior distribution and the maximum likelihood parameter. WL mass differs from true mass due to, for example, the intrinsic ellipticity of sources, correlated or uncorrelated matter and large-scale structure, halo triaxiality and orientation, and merging or irregular morphology. In an ideal scenario without observational or measurement errors, the maximum likelihood estimator is the most accurate, with WL masses biased low by ⟨bM⟩= − 14.6 ± 1.7% on average over the full range M200c > 5 × 1013 M⊙ and z < 1. Due to the stabilising effect of the prior, the biweight, mean, and median estimates are more precise, that is with smaller intrinsic scatter. The scatter decreases with increasing mass and informative priors can significantly reduce the scatter. Halo mass density profiles with a truncation provide better fits to the lensing signal, while the accuracy and precision are not significantly affected. We further investigate the impact of various additional sources of systematic uncertainty on the WL mass estimates, namely the impact of photometric redshift uncertainties and source selection, the expected performance of Euclid cluster detection algorithms, and the presence of masks. Taken in isolation, we find that the largest effect is induced by non-conservative source selection with ⟨bM⟩= − 33.4 ± 1.6%. This effect can be mostly removed with a robust selection. As a final Euclid-like test, we combine systematic effects in a realistic observational setting and find ⟨bM⟩= − 15.5 ± 2.4% under a robust selection. This is very similar to the ideal case, though with a slightly larger scatter mostly due to cluster redshift uncertainty and miscentering.

• Atek, H.; Gavazzi, R.; Weaver, J.R.; Diego, J.M.; Schrabback, T. & Hatch, N.A. [Show all 203 contributors for this article] (2025). Euclid: Early Release Observations – A preview of the Euclid era through a galaxy cluster magnifying lens. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 697. doi: 10.1051/0004-6361/202450776. Full text in Research Archive Show summary

  We present the first analysis of the Euclid Early Release Observations (ERO) program that targets fields around two lensing clusters, Abell 2390 and Abell 2764. We use imaging data from the Visible instrument (VIS) and the Near-Infrared Spectrometer and Photometer (NISP) to produce photometric catalogs for a total of ∼500 000 objects. The imaging data reach a typical depth of 5 σ in the range 25.1–25.4 AB in the NISP bands and 27.1–27.3 AB in the VIS band. Using the Lyman-break method in combination with photometric redshifts, we searched for high-redshift galaxies. We identified 30 Lyman-break galaxy (LBG) candidates at z > 6 and 139 extremely red sources (ERSs), most of which likely lie at lower redshift. The VIS imaging is deeper than the NISP imaging, which means that we can routinely identify high-redshift Lyman-break galaxies at about a magnitude of 3, which reduces contamination by brown dwarf stars and low-redshift galaxies. The difficulty of spatially resolving most of these sources in 0″.3 pix−1 imaging means that it is difficult to distinguish between galaxies and quasars. Spectroscopic follow-up campaigns of these bright sources will help us to constrain the bright end of the ultraviolet galaxy luminosity function and the quasar luminosity function at z > 6, and it will constrain the physical nature of these objects. Additionally, we performed a combined strong- and weak-lensing analysis of A2390, and we show that Euclid will contribute to constraining the virial mass of galaxy clusters better. We also identify optical and near-infrared counterparts of known z > 0.6 clusters in these data. These counterparts exhibit strong-lensing features. This establishes that Euclid can characterize high-redshift clusters. Finally, we provide a glimpse of the ability of Euclid to map the intracluster light out to larger radii than current facilities, which enables us to understand the cluster assembly history better and to map the dark matter distribution. This initial dataset illustrates the diverse spectrum of legacy science that is possible with the Euclid survey.

• Nucita, A.A.; Conversi, L.; Verdier, A.; Franco, A.; Sacquegna, S. & Pöntinen, M. [Show all 144 contributors for this article] (2025). Euclid: Detecting Solar System objects in Euclid images and classifying them using Kohonen self-organising maps. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 694. doi: 10.1051/0004-6361/202451767. Full text in Research Archive Show summary

  The ESA Euclid mission will survey more than 14 000 deg2 of the sky in visible and near-infrared wavelengths, mapping the extragalactic sky to constrain our cosmological model of the Universe. Although the survey focusses on regions further than 15° from the ecliptic, it should allow for the detection of more than about 105 Solar System objects (SSOs). After simulating the expected signal from SSOs in Euclid images acquired with the visible camera (VIS), we describe an automated pipeline developed to detect moving objects with an apparent velocity in the range of 0.1–10″ h−1, typically corresponding to sources in the outer Solar System (from Centaurs to Kuiper-belt objects). In particular, the proposed detection scheme is based on SExtractor software and on applying a new algorithm capable of associating moving objects amongst different catalogues. After applying a suite of filters to improve the detection quality, we study the expected purity and completeness of the SSO detections. We also show how a Kohonen self-organising neural network can be successfully trained (in an unsupervised fashion) to classify stars, galaxies, and SSOs. By implementing an early-stopping method in the training scheme, we show that the network can be used in a predictive way, allowing one to assign the probability of each detected object being a member of each considered class.

• Scognamiglio, D.; Schrabback, T.; Tewes, M.; Gillis, B.; Hoekstra, H. & Huff, E.M. [Show all 247 contributors for this article] (2025). Euclid preparation: LX. The use of HST images as input for weak-lensing image simulations. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 694. doi: 10.1051/0004-6361/202451587. Full text in Research Archive Show summary

  Data from the Euclid space telescope will enable cosmic shear measurements to be carried out with very small statistical errors, necessitating a corresponding level of systematic error control. A common approach to correct for shear biases involves calibrating shape measurement methods using image simulations with known input shear. Given their high resolution, galaxies observed with the Hubble Space Telescope (HST) can, in principle, be utilised to emulate Euclid observations of sheared galaxy images with realistic morphologies. In this work, we employ a GalSim-based testing environment to investigate whether uncertainties in the HST point spread function (PSF) model or in data processing techniques introduce significant biases in weak-lensing (WL) shear calibration. We used single Sérsic galaxy models to simulate both HST and Euclid observations. We then ‘Euclidised’ our HST simulations and compared the results with the directly simulated Euclid-like images. For this comparison, we utilised a moment-based shape measurement algorithm and galaxy model fits. Through the Euclidisation procedure, we effectively reduced the residual multiplicative biases in shear measurements to sub-percent levels. This achievement was made possible by employing either the native pixel scales of the instruments, utilising the Lanczos15 interpolation kernel, correcting for noise correlations, and ensuring consistent galaxy signal-to-noise ratios between simulation branches. Alternatively, a finer pixel scale can be employed alongside deeper HST data. However, the Euclidisation procedure requires further analysis on the impact of the correlated noise, to estimate calibration bias. We found that additive biases can be mitigated by applying a post-deconvolution isotropisation in the Euclidisation set-up. Additionally, we conducted an in-depth analysis of the accuracy of TinyTim HST PSF models using star fields observed in the F606W and F814W filters. We observe that F606W images exhibit a broader scatter in the recovered best-fit focus, compared to those in the F814W filter. Estimating the focus value for the F606W filter in lower stellar density regimes has allowed us to reveal significant statistical uncertainties.

• Ellien, A.; Montes, M.; Ahad, S.L.; Dimauro, P.; Golden-Marx, J.B. & Jimenez-Teja, Y. [Show all 186 contributors for this article] (2025). Euclid: Early Release Observations - The intracluster light of Abell 2390. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 698. doi: 10.1051/0004-6361/202554460. Full text in Research Archive Show summary

  Intracluster light (ICL) provides a record of the dynamical interactions undergone by clusters, giving clues on cluster formation and evolution. Here, we analyse the properties of ICL in the massive cluster Abell 2390 at redshift z = 0.228. Our analysis is based on the deep images obtained by the Euclid mission as part of the Early Release Observations in the near-infrared (YE, JE, HE bands), using the NISP instrument in a 0.75 deg2 field. We subtracted a point–spread function (PSF) model and removed the Galactic cirrus contribution in each band after modelling it with the DAWIS software. We then applied three methods to detect, characterise, and model the ICL and the brightest cluster galaxy (BCG): the CICLE 2D multi-galaxy fitting; the DAWIS wavelet-based multiscale software; and a mask-based 1D profile fitting. We detect ICL out to 600 kpc. The ICL fractions derived by our three methods range between 18% and 36% (average of 24%), while the BCG+ICL fractions are between 21% and 41% (average of 29%), depending on the band and method. A galaxy density map based on 219 selected cluster members shows a strong cluster substructure to the south-east and a smaller feature to the north-west. Ellipticals dominate the cluster's central region, with a centroid offset from the BCG by about 70 kpc and distribution following that of the ICL, while spirals do not trace the entire ICL but rather substructures. The comparison of the BCG+ICL, mass from gravitational lensing, and X-ray maps show that the BCG+ICL is the best tracer of substructures in the cluster. Based on colours, the ICL (out to about 400 kpc) seems to be built by the accretion of small systems (M∼109.5 M⊙), or from stars coming from the outskirts of Milky Way-type galaxies (M∼1010 M⊙). Though Abell 2390 does not seem to be undergoing a merger, it is not yet fully relaxed, since it has accreted two groups that have not fully merged with the cluster core. We estimate that the contributions to the inner 300 kpc of the ICL of the north-west and south-east subgroups are 21% and 9%, respectively.

• Zalesky, L.; McPartland, C. J. R.; Weaver, J.R.; Toft, S.; Sanders, D.B. & Mobasher, B. [Show all 274 contributors for this article] (2025). Euclid preparation: LXI. Cosmic Dawn Survey: ' Pre-launch' multiwavelength catalogues for Euclid Deep Field North and Euclid Deep Field Fornax. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 695. doi: 10.1051/0004-6361/202451857. Full text in Research Archive Show summary

  The Cosmic Dawn Survey (DAWN survey) provides multiwavelength (UV/optical to mid-IR) data across the combined 59 deg2 of the Euclid Deep and Auxiliary fields (EDFs and EAFs). In this work, the first public data release from the DAWN survey is presented. The catalogues made available herein consist of a subset of the full DAWN survey that includes two EDFs: EDF North (EDF-N) and EDF Fornax (EDF-F). Each field has been covered by the ongoing Hawaii Twenty Square Degree Survey (H20), which includes imaging from the CFHT MegaCam in the u filter and from the Subaru Hyper Suprime-Cam (HSC) in the griz filters. Each field has been further covered by Spitzer/IRAC 3.6–4.5µm imaging spanning 10 deg2 and reaching ~25 mag AB (5σ). All present H20 imaging and all publicly available imaging from the aforementioned facilities were combined with the deep Spitzer/IRAC data to create source catalogues spanning a total area of 16.87 deg2 in EDF-N and 2.85 deg2 in EDF-F for this first release. These catalogues are referred to as the ‘pre-launch’ (PL), as Euclid data is not yet public for these fields and therefore it is not included. Photometry was measured from these multiwavelength data using The Farmer, a novel and well validated model-based photometry code. Photometric redshifts and stellar masses were computed using two independent codes for modelling spectral energy distributions: EAZY and LePhare. Photometric redshifts show good agreement with spectroscopic redshifts (σNMAD ~ 0.5, η < 8% at i < 25). Number counts, photometric redshifts and stellar masses were further validated in comparison to the COSMOS2020 catalogue. The DAWN survey PL catalogues are designed to be of immediate use in these two EDFs and will be continuously updated and made available as both new ground-based data and spaced-based data from Euclid are acquired and made public. Future data releases will provide catalogues of all EDFs and EAFs and include Euclid data.

• Kluge, M.; Hatch, N.A.; Montes, M.; Golden-Marx, J.B.; Gonzalez, A.H. & Cuillandre, J.-C. [Show all 188 contributors for this article] (2025). Euclid: Early Release Observations – The intracluster light and intracluster globular clusters of the Perseus cluster. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 697. doi: 10.1051/0004-6361/202450772. Full text in Research Archive Show summary

  We study the intracluster light (ICL) and intracluster globular clusters (ICGCs) in the nearby Perseus cluster of galaxies using Euclid’s Early Release Observations. By modelling the isophotal and iso-density contours, we mapped the distributions and properties of the ICL and ICGCs out to radii of 200-600 kpc (up to ~ 1/3 of the virial radius, depending on the parameter) from the brightest cluster galaxy (BCG). We find that the central 500 kpc of the Perseus cluster hosts 70 000 ± 2800 globular clusters, and 1.7 × 1012 L⊙ of diffuse light from the BCG+ICL in the near-infrared HE. This accounts for 38 ± 6% of the cluster’s total stellar luminosity within this radius. The ICL and ICGCs share a coherent spatial distribution which suggests that they have a common origin or that a common potential governs their distribution. Their contours on the largest scales (>200 kpc) are not centred on the BCG’s core, but are instead offset westwards by 60 kpc towards several luminous cluster galaxies. This offset is opposite to the displacement observed in the gaseous intracluster medium. The radial surface brightness profile of the BCG+ICL is best described by a double Sérsic model, with 68 ± 4% of the HE light contained in the extended, outer component. The transition between these components occurs at ≈60 kpc, beyond which the isophotes become increasingly elliptical and off-centred. Furthermore, the radial ICGC number density profile closely follows the profile of the BCG+ICL only beyond this 60 kpc radius, where we find an average of 60-80 globular clusters per 109 M⊙ of diffuse stellar mass. The BCG+ICL colour becomes increasingly blue with radius, consistent with the stellar populations in the ICL having subsolar metallicities [Fe/H] ~ –0.6 to –1.0. The colour of the ICL, and the specific frequency and luminosity function of the ICGCs suggest that the ICL+ICGCs were tidally stripped from the outskirts of massive satellites with masses of a few ×1010 M⊙, with an increasing contribution from dwarf galaxies at large radii.

• Adamek, J.; Fiorini, B.; Baldi, M.; Brando, G.; Breton, M.-A. & Hassani, Farbod [Show all 270 contributors for this article] (2025). Euclid preparation: LXII. Simulations and non-linearities beyond Lambda cold dark matter. 1. Numerical methods and validation. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 695. doi: 10.1051/0004-6361/202452180. Full text in Research Archive Show summary

  To constrain cosmological models beyond ACDM, the development of the Euclid analysis pipeline requires simulations that capture the non-linear phenomenology of such models. We present an overview of numerical methods and N-body simulation codes developed to study the non-linear regime of structure formation in alternative dark energy and modified gravity theories. We review a variety of numerical techniques and approximations employed in cosmological N-body simulations to model the complex phenomenology of scenarios beyond ACDM. This includes discussions on solving non-linear field equations, accounting for fifth forces, and implementing screening mechanisms. Furthermore, we conduct a code comparison exercise to assess the reliability and convergence of different simulation codes across a range of models. Our analysis demonstrates a high degree of agreement among the outputs of different simulation codes, typically within 2% for the predicted modification of the matter power spectrum and within 4% for the predicted modification of the halo mass function, although some approximations degrade accuracy a bit further. This provides confidence in current numerical methods of modelling cosmic structure formation beyond ACDM. We highlight recent advances made in simulating the non-linear scales of structure formation, which are essential for leveraging the full scientific potential of the forthcoming observational data from the Euclid mission.

• Pearce-Casey, R.; Nagam, B.C.; Wilde, J.; Busillo, V.; Ulivi, L. & Andika, I.T. [Show all 207 contributors for this article] (2025). Euclid: Searches for strong gravitational lenses using convolutional neural nets in Early Release Observations of the Perseus field. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 696. doi: 10.1051/0004-6361/202453152. Full text in Research Archive Show summary

  The Euclid Wide Survey (EWS) is predicted to find approximately 170 000 galaxy-galaxy strong lenses from its lifetime observation of 14 000 deg2 of the sky. Detecting this many lenses by visual inspection with professional astronomers and citizen scientists alone is infeasible. As a result, machine learning algorithms, particularly convolutional neural networks (CNNs), have been used as an automated method of detecting strong lenses, and have proven fruitful in finding galaxy-galaxy strong lens candidates, such that the usage of CNNs in lens identification has increased. We identify the major challenge to be the automatic detection of galaxy-galaxy strong lenses while simultaneously maintaining a low false positive rate, thus producing a pure and complete sample of strong lens candidates from Euclid with a limited need for visual inspection. One aim of this research is to have a quantified starting point on the achieved purity and completeness with our current version of CNN-based detection pipelines for the VIS images of EWS. This work is vital in preparing our CNN-based detection pipelines to be able to produce a pure sample of the >100 000 strong gravitational lensing systems widely predicted for Euclid. We select all sources with VIS IE < 23 mag from the Euclid Early Release Observation imaging of the Perseus field. We apply a range of CNN architectures to detect strong lenses in these cutouts. All our networks perform extremely well on simulated data sets and their respective validation sets. However, when applied to real Euclid imaging, the highest lens purity is just ∼11%. Among all our networks, the false positives are typically identifiable by human volunteers as, for example, spiral galaxies, multiple sources, and artifacts, implying that improvements are still possible, perhaps via a second, more interpretable lens selection filtering stage. There is currently no alternative to human classification of CNN-selected lens candidates. Given the expected ∼105 lensing systems in Euclid, this implies 106 objects for human classification, which while very large is not in principle intractable and not without precedent.

• Castander, F.J.; Fosalba, P.; Stadel, J.; Potter, D.; Carretero, J. & Tallada-Crespí, P. [Show all 369 contributors for this article] (2025). Euclid V. The Flagship galaxy mock catalogue: A comprehensive simulation for the Euclid mission. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 697. doi: 10.1051/0004-6361/202450853. Full text in Research Archive Show summary

  We present the Flagship galaxy mock, a simulated catalogue of billions of galaxies designed to support the scientific exploitation of the Euclid mission. Euclid is a medium-class mission of the European Space Agency optimised to determine the properties of dark matter and dark energy on the largest scales of the Universe. It probes structure formation over more than 10 billion years primarily from the combination of weak gravitational lensing and galaxy clustering data. The breadth of Euclid’s data will also foster a wide variety of scientific analyses. The Flagship simulation was developed to provide a realistic approximation to the galaxies that will be observed by Euclid and used in its scientific exploitation. We ran a state-of-the-art N-body simulation with four trillion particles, producing a lightcone on the fly. From the dark matter particles, we produced a catalogue of 16 billion haloes in one octant of the sky in the lightcone up to redshift z = 3. We then populated these haloes with mock galaxies using a halo occupation distribution and abundance-matching approach, calibrating the free parameters of the galaxy mock against observed correlations and other basic galaxy properties. Modelled galaxy properties include luminosity and flux in several bands, redshifts, positions and velocities, spectral energy distributions, shapes and sizes, stellar masses, star formation rates, metallicities, emission line fluxes, and lensing properties. We selected a final sample of 3.4 billion galaxies with a magnitude cut of HE < 26, where we are complete. We have performed a comprehensive set of validation tests to check the similarity to observational data and theoretical models. In particular, our catalogue is able to closely reproduce the main characteristics of the weak lensing and galaxy clustering samples to be used in the mission main cosmological analysis. Moreover, given its depth and completeness, this new galaxy mock also provides the community with a powerful tool for developing a wide range of scientific analyses beyond the Euclid mission.

• Jahnke, K.; Gillard, W.; Schirmer, M.; Ealet, A.; Maciaszek, T. & Prieto, E. [Show all 440 contributors for this article] (2025). Euclid: III. The NISP Instrument. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 697. doi: 10.1051/0004-6361/202450786. Full text in Research Archive Show summary

  The Near-Infrared Spectrometer and Photometer (NISP) on board the Euclid satellite provides multiband photometry and R ≳ 450 slitless grism spectroscopy in the 950–2020 nm wavelength range. In this reference article, we illuminate the background of NISP’s functional and calibration requirements, describe the instrument’s integral components, and provide all its key properties. We also sketch the processes needed to understand how NISP operates and is calibrated as well as its technical potentials and limitations. Links to articles providing more details and the technical background are included. The NISP’s 16 HAWAII-2RG (H2RG) detectors with a plate scale of 0″.3 pixel−1 deliver a field of view of 0.57 deg2. In photometric mode, NISP reaches a limiting magnitude of ~24.5 AB mag in three photometric exposures of about 100 s in exposure time for point sources and with a S/N of five. For spectroscopy, NISP’s pointsource sensitivity is a signal-to-noise ratio = 3.5 detection of an emission line with flux ~2 × 10−16 erg s−1 cm−2 integrated over two resolution elements of 13.4 Å in 3 × 560 s grism exposures at 1.6 µm (redshifted Hα). Our calibration includes on-ground and in-flight characterisation and monitoring of the pixel-based detector baseline, dark current, non-linearity, and sensitivity to guarantee a relative photometric accuracy better than 1.5% and a relative spectrophotometry better than 0.7%. The wavelength calibration must be accurate to 5 Å or better. The NISP is the state-of-the-art instrument in the near-infrared for all science beyond small areas available from HST and JWST – and it represents an enormous advance from any existing instrumentation due to its combination of field size and high throughput of telescope and instrument. During Euclid’s six-year survey covering 14 000 deg2 of extragalactic sky, NISP will be the backbone in determining distances of more than a billion galaxies. Its near-infrared data will become a rich reference imaging and spectroscopy data set for the coming decades.

• Weaver, J.R.; Taamoli, S.; McPartland, C.J.R.; Zalesky, L.; Allen, N. & Toft, S. [Show all 188 contributors for this article] (2025). Euclid: Early Release Observations – NISP-only sources and the search for luminous z = 6–8 galaxies. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 697. doi: 10.1051/0004-6361/202450794. Full text in Research Archive Show summary

  This paper presents a search for high redshift galaxies from the Euclid Early Release Observations program ‘Magnifying Lens.’ The 1.5 deg2 area covered by the twin Abell lensing cluster fields is comparable in size to the few other deep near-infrared surveys such as COSMOS, and so provides an opportunity to significantly increase known samples of rare UV-bright galaxies at z ≈ 6–8 (MUV ≲ −22). Beyond their still uncertain role in reionisation, these UV-bright galaxies are ideal laboratories from which to study galaxy formation and constrain the bright-end of the UV luminosity function. Of the 501 994 sources detected from a combined YE, JE, and HE NISP detection image, 168 do not have any appreciable VIS/IE flux. These objects span a range in spectral colours, separated into two classes: 139 extremely red sources; and 29 Lyman-break galaxy candidates. Best-fit redshifts and spectral templates suggest the former is composed of both z ≳ 5 dusty star-forming galaxies and z ≈ 1–3 quiescent systems. The latter is composed of more homogeneous Lyman-break galaxies at z ≈ 6–8. In both cases, contamination by L- and T-type dwarfs cannot be ruled out with Euclid images alone. Additional contamination from instrumental persistence is investigated using a novel time series analysis. This work lays the foundation for future searches within the Euclid Deep Fields, where thousands more z ≳ 6 Lyman-break systems and extremely red sources will be identified.

• Tessore, N.; Joachimi, B.; Loureiro, A.; Hall, A.; Cañas-Herrera, G. & Tutusaus, I. [Show all 268 contributors for this article] (2025). Euclid preparation: LIX. Angular power spectra from discrete observations. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 694. doi: 10.1051/0004-6361/202452018. Full text in Research Archive Show summary

  In this paper we present the framework for measuring angular power spectra in the Euclid mission. The observables in galaxy surveys, such as galaxy clustering and cosmic shear, are not continuous fields, but discrete sets of data, obtained only at the positions of galaxies. We show how to compute the angular power spectra of such discrete data sets, without treating observations as maps of an underlying continuous field that is overlaid with a noise component. This formalism allows us to compute the exact theoretical expectations for our measured spectra, under a number of assumptions that we track explicitly. In particular, we obtain exact expressions for the additive biases (‘shot noise’) in angular galaxy clustering and cosmic shear. For efficient practical computations, we introduce a spin-weighted spherical convolution with a well-defined convolution theorem, which allows us to apply exact theoretical predictions to finite-resolution maps, including HEALPix. When validating our methodology, we find that our measurements are biased by less than 1% of their statistical uncertainty in simulations of Euclid’s first data release.

• O'Riordan, C.M.; Oldham, L.J.; Nersesian, A.; Li, T.; Collett, T.E. & Sluse, D. [Show all 198 contributors for this article] (2025). Euclid: A complete Einstein ring in NGC 6505. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 694. doi: 10.1051/0004-6361/202453014. Full text in Research Archive Show summary

  We report the discovery of a complete Einstein ring around the elliptical galaxy NGC 6505, at z = 0.042. This is the first strong gravitational lens discovered in Euclid and the first in an NGC object from any survey. The combination of the low redshift of the lens galaxy, the brightness of the source galaxy (IE = 18.1 lensed, IE = 21.3 unlensed), and the completeness of the ring make this an exceptionally rare strong lens, unidentified until its observation by Euclid. We present deep imaging data of the lens from the Euclid Visible Camera (VIS) and Near-Infrared Spectrometer and Photometer (NISP) instruments, as well as resolved spectroscopy from the Keck Cosmic Web Imager (KCWI). The Euclid imaging in particular presents one of the highest signal-to-noise ratio optical/near-infrared observations of a strong gravitational lens to date. From the KCWI data we measure a source redshift of z = 0.406. Using data from the Dark Energy Spectroscopic Instrument (DESI) we measure a velocity dispersion for the lens galaxy of σ⋆ = 303 ± 15 km s−1. We model the lens galaxy light in detail, revealing angular structure that varies inside the Einstein ring. After subtracting this light model from the VIS observation, we model the strongly lensed images, finding an Einstein radius of 2.″5, corresponding to 2.1 kpc at the redshift of the lens. This is small compared to the effective radius of the galaxy, Reff ∼ 12.″3. Combining the strong lensing measurements with analysis of the spectroscopic data we estimate a dark matter fraction inside the Einstein radius of fDM = (11.1−3.5+5.4)% and a stellar initial mass-function (IMF) mismatch parameter of αIMF = 1.26−0.08+0.05, indicating a heavier-than-Chabrier IMF in the centre of the galaxy.

• Cropper, M.S.; Al-Bahlawan, A.; Amiaux, J.; Awan, S.; Azzollini, R. & Benson, K. [Show all 434 contributors for this article] (2025). Euclid II. The VIS instrument. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 697. doi: 10.1051/0004-6361/202450996. Full text in Research Archive Show summary

  This paper presents the specification, design, and development of the Visible Camera (VIS) on the European Space Agency’s Euclid mission. VIS is a large optical-band imager with a field of view of 0.54 deg2 sampled at 0″.1 with an array of 609 Megapixels and a spatial resolution of 0″.18. It will be used to survey approximately 14 000 deg2 of extragalactic sky to measure the distortion of galaxies in the redshift range z = 0.1–1.5 resulting from weak gravitational lensing, one of the two principal cosmology probes leveraged by Euclid. With photometric redshifts, the distribution of dark matter can be mapped in three dimensions, and the extent to which this has changed with look-back time can be used to constrain the nature of dark energy and theories of gravity. The entire VIS focal plane will be transmitted to provide the largest images of the Universe from space to date, specified to reach mAB ≥ 24.5 with a signal-to-noise ratio S/N ≥ 10 in a single broad IE ≃ (r + i + z) band over a six-year survey. The particularly challenging aspects of the instrument are the control and calibration of observational biases, which lead to stringent performance requirements and calibration regimes. With its combination of spatial resolution, calibration knowledge, depth, and area covering most of the extra-Galactic sky, VIS will also provide a legacy data set for many other fields. This paper discusses the rationale behind the conception of VIS and describes the instrument design and development, before reporting the prelaunch performance derived from ground calibrations and brief results from the inorbit commissioning. VIS should reach fainter than mAB = 25 with S/N ≥ 10 for galaxies with a full width at half maximum of 0″. 3 in a 1″.3 diameter aperture over the Wide Survey, and mAB ≥ 26.4 for a Deep Survey that will cover more than 50 deg2. The paper also describes how the instrument works with the Euclid telescope and survey, and with the science data processing, to extract the cosmological information.

• Elkhashab, M.Y.; Bertacca, D.; Porciani, C.; Salvalaggio, J.; Aghanim, N. & Amara, A. [Show all 254 contributors for this article] (2025). Euclid preparation: LXIX. The impact of relativistic redshift-space distortions on two-point clustering statistics from the Euclid wide spectroscopic survey. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 697. doi: 10.1051/0004-6361/202452480. Full text in Research Archive Show summary

  Measurements of galaxy clustering are affected by redshift-space distortions (RSDs). Peculiar velocities, gravitational lensing, and other light-cone projection effects modify the observed redshifts, fluxes, and sky positions of distant light sources. We determined which of these effects leave a detectable imprint on several two-point clustering statistics to be extracted from the Euclid wide spectroscopic survey (EWSS) on large scales. We generated 140 mock galaxy catalogues with the survey geometry and selection function of the EWSS and made use of the LIGER (LIght cones with GEneral Relativity) method to account for a variable number of relativistic RSDs to linear order in the cosmological perturbations. We estimated different two-point clustering statistics from the mocks and used the likelihood-ratio test to calculate the statistical significance with which the EWSS could reject the null hypothesis that certain relativistic projection effects can be neglected in the theoretical models. We find that the combined effects of lensing magnification and convergence imprint characteristic signatures on several clustering observables. Their signal-to-noise ratio (S/N) ranges between 2.5 and 6 (depending on the adopted summary statistic) for the highest-redshift galaxies in the EWSS. The corresponding feature due to the peculiar velocity of the Sun is measured with a S/N of order one or two. The multipoles of the power spectrum from the catalogues that include all relativistic effects reject the null hypothesis that RSDs are only generated by the variation in the peculiar velocity along the line of sight with a significance of 2.9 standard deviations. As a by-product of our study, we demonstrate that the mixing-matrix formalism to model finite-volume effects in the multipole moments of the power spectrum can be robustly applied to surveys made of several disconnected patches. Our results indicate that relativistic RSDs, in particular the contribution from weak gravitational lensing, cannot be disregarded when modelling two-point clustering statistics extracted from the EWSS.

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