Sijing Shen

Full publication list can be found  here .

• Rosselli-Calderon, Alejandra; Stewart, Julia; Shen, Sijing; Chakrabarti, Sukanya; Soares-Furtado, Melinda & Ramirez-Ruiz, Enrico (2026). Chemical Enrichment of Metal-poor Stars Orbiting Massive Black Hole Companions. The Astrophysical Journal (ApJ). ISSN 0004-637X. 997(1). doi: 10.3847/1538-4357/ae28c6. Full text in Research Archive Show summary

  There are millions of undetected black holes wandering through our galaxy. Observatories like Chandra, LIGO, and more recently, Gaia, have provided valuable insights into the configurations of these elusive objects when residing in binary systems. Motivated by these advances, we study, for the first time, the enhanced accretion of metals from the interstellar medium (ISM) onto low-mass companions in binary systems with highly unequal mass ratios, utilizing a series of hydrodynamical simulations. Our study demonstrates that a stellar companion’s metal accretion history from the ISM alone, from its formation to the present, can significantly influence its surface abundance, especially when enhanced by a massive black hole companion. However, this effect is likely only measurable in stars that are still on the main sequence. Once a stellar companion evolves off the main sequence, similar to what has been observed with the Gaia BH3 companion, the initial dredge-up process is likely to erase any excess surface abundance resulting from the metals that were accreted. As we discover more unequal mass ratio binary systems, it is crucial to understand how the observed metallicity of Sun-like companions may differ from their birth metallicity, especially if they are not yet evolved.

• Piacitelli, Daniel R.; Brooks, Alyson M.; Christensen, Charlotte; Sanchez, N. Nicole; Faerman, Yakov & Shen, Sijing [Show all 10 contributors for this article] (2025). Marvelous Metals: Surveying the Circumgalactic Medium of Simulated Dwarf Galaxies. The Astrophysical Journal (ApJ). ISSN 0004-637X. 993(2). doi: 10.3847/1538-4357/ae06a0. Full text in Research Archive Show summary

  Dwarf galaxies are uniquely sensitive to feedback processes and known to experience substantial mass and metal loss from their disks. Here, we investigate the circumgalactic medium (CGM) of 64 isolated dwarf galaxies ( 6.0 < log ( M * / M ⊙ ) < 9.5 ) at z = 0 from the Marvel-ous Dwarfs and Marvelous Massive Dwarfs simulations. Our galaxies produce column densities broadly consistent with current observations. We investigate these column densities in the context of mass and metal retention rates, and CGM physical properties. We find 48% ± 11% of all baryons within R 200 c reside in the CGM, with ∼70% of CGM mass existing in a warm gas phase, 10 4.5  <  T  < 10 5.5 K, that dominates beyond r / R 200 c  ∼ 0.5. The warm and cool (10 4.0  <  T  < 10 4.5 K) gas phases each retain 5%–10% of metals formed by the dwarf galaxy. The significant fraction of mass and metals residing in the warm CGM phase provides an interpretation for the lack of z  ∼ 0 low ion detections beyond b / R 200 c  ∼ 0.5, as the majority of mass in this region exists in higher ions. We find a weak correlation between galaxy mass and total CGM metal retention despite the fraction of metals lost from the halo increasing from ∼10% to >40% toward lower masses. Our findings highlight the CGM (particularly its warm phase) as a key reservoir of mass and metals for dwarf galaxies across stellar masses, underscoring its importance in understanding the baryon cycle in the low-mass regime. Finally, we provide individual simulated galaxy properties and quantify the fraction of UV-observable mass to support future observational programs aimed at performing a metal budget around dwarf galaxies.

• Nyhagen, Camilla Thune; Schimek, Alice; Cicone, Claudia; Decataldo, Davide & Shen, S. (2025). A theoretical investigation of far-infrared fine structure lines at z<6 and of the origin of the [O III] 88 μm/[C II]158 μm enhancement. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 702. doi: 10.1051/0004-6361/202452718. Full text in Research Archive Show summary

  The far-infrared (FIR) fine structure lines [C  II ] 158 μm , [O  III ] 88 μm , [N  II ] 122 μm , and [N  III ] 57 μm are excellent tools for probing the physical conditions of the interstellar medium (ISM). The [O  III ] 88 μm /[C  II ] 158 μm and [O  III ] 88 μm /[N  II ] 122 μm luminosity ratios have shown to be promising tracers of the ionisation state and gas-phase metallicity of the ISM. Observations of galaxies at redshift z  > 6 show unusually high [O  III ] 88 μm /[C  II ] 158 μm luminosity ratios compared to local sources. The origin of the enhanced ratios has been investigated in the literature with different theoretical modelling approaches. However, no model has to date successfully managed to match the observed emission from both [O  III ] 88 μm and [C  II ] 158 μm , as well as their ratio. For this study we used C LOUDY to model the [C  II ] 158 μm , [O  III ] 88 μm , [N  II ] 122 μm , and [N  III ] 57 μm emission lines of P ONOS , a high-resolution ( m gas  = 883.4  M ⊙ ) cosmological zoom-in simulation of a galaxy at redshift z  = 6.5, which is post-processed using KRAMSES - RT . We modify carbon, nitrogen, and oxygen abundances in our C LOUDY models to obtain C/O and N/O abundance ratios respectively lower and higher than solar, more in line with recent high- z observational constraints. We find [O  III ] 88 μm /[C  II ] 158 μm luminosity ratios that are a factor of ∼5 higher compared to models assuming solar abundances. Additionally, we find an overall better agreement of the simulation with high- z observational constraints of the [C  II ] 158 μm -SFR and [O  III ] 88 μm -SFR relations. This shows that a lower C/O abundance ratio is essential to reproduce the enhanced [O  III ] 88 μm /[C  II ] 158 μm luminosity ratios observed at z  > 6. By assuming a super-solar N/O ratio, motivated by recent z  > 6 JWST observations, our models yield an [O  III ] 88 μm /[N  II ] 122 μm ratio of 1.3, which, according to current theoretical models, would be more appropriate for a galaxy with a lower ionisation parameter than the one we estimated for P ONOS . Most current simulations adopt solar abundance patterns that are not adequate for recently observed high- z predictions. Our results showcase the importance of theoretical modelling efforts, coupled with high-resolution zoom-in simulations, and with parallel multi-tracer observations to understand the physical and chemical conditions of the ISM at z  > 6.

• Zimmermann, Tim; Marsh, D.J.E.; Rogers, K.K.; Winther, Hans Arnold & Shen, Sijing (2025). Interference in fuzzy dark matter filaments: Idealized models and statistics. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 696. doi: 10.1051/0004-6361/202554232. Full text in Research Archive Show summary

  Fuzzy (wave) dark matter (FDM), the dynamical model underlying an ultralight bosonic dark matter species, produces a rich set of nongravitational signatures that distinguishes it markedly from the phenomenologically related warm (particle) dark matter (WDM) scenario. The emergence of extended interference fringes hosted by cosmic filaments is one such phenomenon reported by cosmological simulations, and a detailed understanding of such may strengthen existing limits on the boson mass but also break the degeneracy with WDM, and provide a unique fingerprint of interference in cosmology. In this paper, we provide initial steps toward this goal. In particular, we show in a bottom-up approach, how the presence of interference in an idealized filament population can lead to a non-suppressive feature in the matter power spectrum – an observation supported by cosmological FDM simulations. To this end, we build on a theoretically motivated and numerically observed steady-state approximation for filaments and express the equilibrium dynamics of such in an expansion of FDM eigenstates. We optimize the size of the expansion by incorporating classical phase-space information. Ellipsoidal collapse considerations were used to construct a fuzzy filament mass function which, together with the reconstructed FDM wave function, allowed us to efficiently compute the one-filament power spectrum. We showcase our non-perturbative interference model for a selection of boson masses and confirm our approach is able to produce the matter power boost observed in fully cosmological FDM simulations. More precisely, we find an excess in correlation between the spatial scale associated with the FDM ground state and the quantum pressure scale. We speculate about applications of this effect in data analysis.

• Schimek, Alice; Decataldo, Davide; Shen, Sijing; Cicone, Claudia; Baumschlager, Bernhard & Kampen, E van [Show all 13 contributors for this article] (2024). High resolution modelling of [CII], [CI], [OIII], and CO line emission from the interstellar medium and circumgalactic medium of a star-forming galaxy at z ∼ 6.5. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 682. doi: 10.1051/0004-6361/202346945. Full text in Research Archive Show summary

  The circumgalactic medium (CGM) is a crucial component of galaxy evolution, but thus far its physical properties are highly unconstrained. As of yet, no cosmological simulation has reached convergence when it comes to constraining the cold and dense gas fraction of the CGM. Such components are also challenging to observe directly, as they require sub-millimetre (sub-mm) instruments with a high sensitivity to extended and mostly diffuse emission. We present a state-of-the-art theoretical effort at modelling the [CII] 158 μm, [CI](1−0) 609 μm, [CI](2−1) 370 μm, CO(3−2) 867 μm, and [OIII] 88 μm line emissions that arise from the interstellar medium (ISM) and CGM of galaxies, with the goal of studying the contribution from different cold (T < 104 K) components of galaxy halos. We used the high-resolution cosmological zoom-in simulation PONOS (mgas = 883.4 M⊙), which represents a typical star-forming galaxy system at z = 6.5, composed of a main disc with stellar mass M* = 2 × 109 M⊙ that is undergoing a major merger. We adopted different modelling approaches based on the photoionisation code CLOUDY. Our fiducial model uses radiative transfer post-processing with RAMSES-RT and KROME (KRAMSES-RT) to create more realistic far-ultraviolet (FUV) radiation fields, which we then compared to other sub-grid modelling approaches adopted in the literature. We find significant differences in the luminosity and in the contribution of different gas phases and galaxy components between the different modelling approaches. [CII] is the least model-dependant gas tracer, while [CI](1−0) and CO(3−2) are very model-sensitive. In all models, we find a significant contribution to the emission of [CII] (up to ∼10%) and [OIII] (up to ∼21%) from the CGM. Our fiducial global radiative transfer (RT) model produces a lower density, T ∼ 104 K tail of [CII] emission that is not seen in the other more simplistic models and that resides entirely in the CGM, ionised by the FUV background and producing the extended halos observed in [CII] at high-z. Notably, [CII] and [OIII] trace different regions of the CGM: [CII] arises from an accreting filament and from the tidal tails connecting the main disc and its merging satellites, while [OIII] traces a puffy halo surrounding the main disc, probably linked to supernova feedback. We discuss our results in the context of sub-mm observations. Using simulated spectra and mock maps, we show that, despite the rather compact angular extent of PONOS’s CGM, deep ALMA observations would not detect this component, even in [CII] which is the brightest available tracer. Instead, a next generation single-dish observatory such as the Atacama Large Aperture Submillimeter Telescope (AtLAST) could detect PONOS’ CGM in [CII] at a high signal-to-noise ratio, and possibly even in [OIII].

• Taffoni, Giuliano; Mignone, Andrea; Tornatore, Luca; Sciacca, Eva; Guarrasi, Massimiliano & Lapenta, Giovanni [Show all 27 contributors for this article] (2024). Redesign of astrophysical codes for exascale computing: the SPACE experience. Proceedings of SPIE, the International Society for Optical Engineering. ISSN 0277-786X. 13101. doi: 10.1117/12.3020361. Full text in Research Archive Show summary

  High Performance Computing based simulations are crucial in Astrophysics and Cosmology, helping scientists investigate and understand complex astrophysical phenomena. Taking advantage of Exascale computing capabilities is essential for these efforts. However, the unprecedented architectural complexity of exascale systems impacts simulation codes. The SPACE Center of Excellence aims to re-engineer key astrophysical codes to adapt to these new computational challenges by adopting innovative programming paradigms and software solutions. Through co-design activities, SPACE brings together scientists, code developers, HPC experts, hardware manufacturers, and software developers. This collaboration enhances exascale astrophysics and cosmology applications, promoting the use of exascale and post-exascale computing capabilities. Additionally, SPACE addresses high-performance data analysis for the massive data outputs from exascale simulations, using machine learning and visualization tools. The project facilitates application deployment across platforms by focusing on code repositories and data sharing, integrating European astrophysical communities around exascale computing with standardized software and data protocols. In this paper, we present the SPACE Center of Excellence and the preliminary results achieved by the project.

• Wadsley, James W.; Baumschlager, Bernhard & Shen, Sijing (2024). TREVR2: Illuminating fast N log_2 N radiative transfer. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 528(2), p. 3767–3781. doi: 10.1093/mnras/stae209. Full text in Research Archive Show summary

  ABSTRACT We present trevr2 (Tree-based REVerse Ray Tracing 2), a fast, general algorithm for computing the radiation field, suitable for both particle and mesh codes. It is designed to self-consistently evolve chemistry for zoomed-in astrophysical simulations, such as cosmological galaxies with both internal sources and prescribed background radiation, rather than large periodic volumes. Light is propagated until absorbed, with no imposed speed limit other than those due to opacity changes (e.g. ionization fronts). trevr2 searches outward from receiving gas in discrete directions set by the healpix algorithm (unlike its slower predecessor trevr), accumulating optical depth and adding the flux due to sources combined into progressively larger tree cells with distance. We demonstrate Nactivelog2N execution time with absorption and many sources. This allows multiband radiative transfer costs comparable to tree-based gravity and hydrodynamics, and the usual speed-up when active particles evolve on individual time-steps. Sources embedded in non-homogeneous absorbing material introduce systematic errors. We introduce transmission averaging instead of absorption averaging which dramatically reduces these systematic effects. We outline other ways to address systematics including an explicit complex source model. We demonstrate the overall performance of the method via a set of astrophysical test problems.

• Schimek, Alice; Cicone, Claudia; Shen, Sijing; Decataldo, Davide; Klaassen, P. & Mayer, L. (2024). Constraining the physical properties of gas in high-z galaxies with far-infrared and submillimetre line ratios. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 687. doi: 10.1051/0004-6361/202449903. Full text in Research Archive
• Decataldo, Davide; Shen, Sijing; Mayer, Lucio; Baumschlager, Bernhard & Madau, Piero (2024). The origin of cold gas in the circumgalactic medium. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 685. doi: 10.1051/0004-6361/202346972. Full text in Research Archive Show summary

  Context. The presence of cold gas (T ≲ 104 K) in the circumgalactic medium (CGM) of galaxies has been confirmed in observations and in high-resolution simulations, but its origin is still a puzzle. Possible mechanisms are cold accretion from the intergalactic medium (IGM), clumps embedded in outflows and transported from the disk, and gas detaching from the hot CGM phase via thermal instabilities. Aims. In this work we characterize the history of cold CGM gas in order to identify the dominant origin channels at different evolutionary stages of the main galaxy. Methods. To this end, we tracked gas particles in different snapshots of the smoothed particle hydrodynamics (SPH) cosmological zoom-in simulation Eris2k. We performed a backward tracking of cold gas, starting from different redshifts until we could identify one of the followings origins for the particle: cold inflow, ejection from the disk, cooling down in situ, or stripping from a satellite. We also performed a forward tracking of gas in different components of the galaxy (such as the disk and outflows). Results. We find a clear transition between two epochs. For z > 2, most cold gas (up to 80%) in the CGM comes from cold accretion streams as the galaxy is accreting in the cold mode from the IGM. At lower z, gas either cools down in situ after several recycles (with 10–20% of the gas cooling in outflow), or it is ejected directly from the disk (up to 30%). Outflows have a major contribution to the cold CGM gas budget at z < 1, with almost 50% of the hot gas cooling in outflow. Finally, we discuss possible mechanisms for CGM cooling, showing that the thermally unstable gas with tcool/tff < 1 (precipitation-regulated feedback) is abundant up to r ∼ 100 kpc and cooling times are shorter than 50 Myr for densities n > 10−2 cm−3.

• Baumschlager, Bernhard; Shen, Sijing & Wadsley, James (2024). Spectral reconstruction for radiation hydrodynamic simulations of galaxy evolution. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 691. doi: 10.1051/0004-6361/202348164. Full text in Research Archive Show summary

  Radiation from stars and active galactic nuclei (AGN) plays an important role in galaxy formation and evolution, and profoundly transforms the intergalactic, circumgalactic, and interstellar medium (IGM, CGM, and ISM). On-the-fly radiative transfer (RT) has started being incorporated in cosmological simulations, but the complex evolving radiation spectra are often crudely approximated with a small number of broad bands with piece-wise constant intensity and a fixed photo-ionisation cross-section. Such a treatment is unable to capture the changes to the spectrum as light is absorbed while it propagates through a medium with non-zero opacity. This can lead to large errors in photo-ionisation and heating rates. In this work we present a novel approach of discretising the radiation field at discrete photon energies, at the edges of the typically used photo-ionising bands, in order to capture the power-law slope of the radiation field. In combination with power-law approximations for the photo-ionisation cross-sections, this model allows us to self-consistently combine radiation from sources with different spectra and accurately follow the ionisation states of primordial and metal species through time. The method is implemented in GASOLINE2 in connection with TREVR2, a fast reverse ray tracing algorithm with 𝒪(Nactive log2 N) scaling. We compare our new piece-wise power-law reconstruction to the piece-wise constant method in calculating the primordial chemistry photo-ionisation and heating rates under an evolving UV background (UVB) and stellar spectrum, and find that our method reduces errors significantly, by up to two orders of magnitude in the case of HeII ionisation. We apply our new spectral reconstruction method in RT post-processing of a cosmological zoom-in simulation, MUGS2 g1536, including radiation from stars and a live UVB, and find a significant increase in total neutral hydrogen (HI) mass in the ISM and the CGM due to shielding of the UVB and a low escape fraction of the stellar radiation. This demonstrates the importance of RT and an accurate spectral approximation in simulating the CGM-galaxy ecosystem.

• Hagedorn, Jan Bendix; Cicone, Claudia; Sarzi, M.; Saintonge, A.; Severgnini, P. & Vignali, C. [Show all 10 contributors for this article] (2024). Molecular gas scaling relations for local star-forming galaxies in the low-M* regime. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 687. doi: 10.1051/0004-6361/202449773. Full text in Research Archive Show summary

  We derived molecular gas fractions (fmol = Mmol/M*) and depletion times (τmol = Mmol/SFR) for 353 galaxies representative of the local star-forming population with 108.5 M⊙ < M* < 1010.5 M⊙ drawn from the ALLSMOG and xCOLDGASS surveys of CO(2−1) and CO(1−0) line emission. By adding constraints from low-mass galaxies and upper limits for CO non-detections, we find the median molecular gas fraction of the local star-forming population to be constant at log fmol = −0.99−0.19+0.22, challenging previous reports of increased molecular gas fractions in low-mass galaxies. Above M* ∼ 1010.5 M⊙, we find the fmol versus M* relation to be sensitive to the selection criteria for star-forming galaxies. We tested the robustness of our results against different prescriptions for the CO-to-H2 conversion factor and different selection criteria for star-forming galaxies. The depletion timescale τmol weakly depends on M*, following a power law with a best-fit slope of 0.16 ± 0.03. This suggests that small variations in specific star formation rate (sSFR = SFR/M*) across the local main sequence of star-forming galaxies with M* < 1010.5 M⊙ are mainly driven by differences in the efficiency of converting the available molecular gas into stars. We tested these results against a possible dependence of fmol and τmol on the surrounding (group) environment of the targets by splitting them into centrals, satellites, and isolated galaxies, and find no significant variation between these populations. We conclude that the group environment is unlikely to have a large systematic effect on the molecular gas content of star-forming galaxies in the local Universe.

• Lee, Minju; Schimek, Alice; Cicone, Claudia; Andreani, Paola; Popping, Gergo & Sommovigo, Laura [Show all 32 contributors for this article] (2024). Atacama Large Aperture Submillimeter Telescope (AtLAST) science: The hidden circumgalactic medium. Open Research Europe. 4. doi: 10.12688/openreseurope.17452.1. Full text in Research Archive Show summary

  Our knowledge of galaxy formation and evolution has incredibly progressed through multi-wavelength observational constraints of the interstellar medium (ISM) of galaxies at all cosmic epochs. However, little is known about the physical properties of the more diffuse and lower surface brightness reservoir of gas and dust that extends beyond ISM scales and fills dark matter haloes of galaxies up to their virial radii, the circumgalactic medium (CGM). New theoretical studies increasingly stress the relevance of the latter for understanding the feedback and feeding mechanisms that shape galaxies across cosmic times, whose cumulative effects leave clear imprints into the CGM. Recent studies are showing that a – so far unconstrained – fraction of the CGM mass may reside in the cold (T < 104 K) molecular and atomic phase, especially in high-redshift dense environments. These gas phases, together with the warmer ionised phase, can be studied in galaxies from z ∼ 0 to z ∼ 10 through bright far-infrared and sub-millimeter emission lines such as [C ii] 158µm, [O iii] 88 µm, [C I] 609µm, [C i] 370µm, and the rotational transitions of CO. Imaging such hidden cold CGM can lead to a breakthrough in galaxy evolution studies but requires a new facility with the specifications of the proposed Atacama Large Aperture Submillimeter Telescope (AtLAST). In this paper, we use theoretical and empirical arguments to motivate future ambitious CGM observations with AtLAST and describe the technical requirements needed for the telescope and its instrumentation to perform such science.

• Larsen, Ann-Cecilie; Tveten, Gry Merete; Renstrøm, Therese; Utsunomiya, H.; Algin, E. & Ari-Izumi, T. [Show all 31 contributors for this article] (2023). New experimental constraint on the W 185 (n,γ) W 186 cross section. Physical Review C. ISSN 2469-9985. 108(2). doi: 10.1103/PhysRevC.108.025804. Full text in Research Archive
• Arroyave, Isabel Montoya; Cicone, Claudia; Makroleivaditi, E.; Weiss, A.; Lundgren, A. & Severgnini, P. [Show all 11 contributors for this article] (2023). A sensitive APEX and ALMA CO(1-0), CO(2-1), CO(3-2), and [CI](1-0) spectral survey of 40 local (ultra-)luminous infrared galaxies. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 673. doi: 10.1051/0004-6361/202245046. Full text in Research Archive Show summary

  We present a high sensitivity, ground-based spectral line survey of low- J carbon monoxide (CO( J up  →  J up  − 1) with J up  = 1, 2, 3) and neutral carbon [CI] 3 P 1 − 3 P 0 ([CI](1–0)) in 36 local ultra-luminous infrared galaxies (ULIRGs) and 4 additional LIRGs, all of which have previous Herschel OH 119 μm observations. The study is based on new single-dish observations conducted with the Atacama Pathfinder Experiment (APEX) and complemented with archival APEX and Atacama Large Millimeter Array (ALMA and ACA) data. Our methods are optimized for a multi-tracer study of the total molecular line emission from these ULIRGs, including any extended low-surface-brightness components. We find a tight correlation between the CO and [CI] line luminosities, which suggests that the emission from CO(1–0) (and CO(2–1)) arises from similar regions as the [CI](1–0), at least when averaged over galactic scales. By using [CI] to compute molecular gas masses, we estimate a median CO-to-H 2 conversion factor of ⟨ α CO ⟩ = 1.7 ± 0.5  M ⊙ (K km s −1 pc 2 ) −1 for ULIRGs. We derive median galaxy-integrated CO line ratios of 〈 r 21 〉 =  L CO(2-1) ′ / L CO(1-0) ′  = 1.09, 〈 r 31 〉 =  L CO(3-2) ′ / L CO(1-0) ′  = 0.76, and 〈 r 32 〉 =  L CO(3-2) ′ / L CO(2-1) ′  = 0.76, significantly higher than normal star-forming galaxies, confirming the exceptional molecular gas properties of ULIRGs. We find that the r 21 and r 32 ratios are poor tracers of CO excitation in ULIRGs, while r 31 shows a positive trend with L IR and star formation rates and a negative trend with the H 2 gas depletion timescales ( τ dep ). Our investigation of CO line ratios as a function of gas kinematics shows no clear trends, except for a positive relation between r 21 and σ v , which can be explained by CO opacity effects. These ULIRGs are also characterized by high L [CI](1-0) ′ / L CO(1-0) ′ ratios, with a measured median value of ⟨ r CICO ⟩ = 0.18, higher than values from previous interferometric studies that were affected by missing [CI] line flux. The r CICO values do not show a significant correlation with any of the galaxy properties investigated, including OH outflow velocities and equivalent widths. We find that the widths of [CI](1–0) lines are ∼10% smaller than those of CO lines, and that this discrepancy becomes more significant in ULIRGs with broad lines ( σ v  &gt; 150 km s −1 ) and when considering the high- v wings of the lines. This suggests that the low optical depth of [CI] can challenge its detection in diffuse, low-surface-brightness outflows and, therefore, its use as a tracer of CO-dark H 2 gas in these components. Finally, we find that higher L AGN are associated with longer τ dep , consistent with the hypothesis that active galactic nucleus feedback may reduce the efficiency of star formation. Our study highlights the need for sensitive single-dish multi-tracer H 2 surveys of ULIRGs that are able to recover the flux that is missed by interferometers, especially in the high-frequency lines such as [CI]. The Atacama Large Aperture Submillimeter Telescope (AtLAST) will be transformational for this field.

• Wissing, Robert Bleckert Nils Håkan & Shen, Sijing (2023). Numerical dependencies of the galactic dynamo in isolated galaxies with SPH. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 673. doi: 10.1051/0004-6361/202244753. Full text in Research Archive Show summary

  Simulating and evolving magnetic fields within global galaxy simulations provides a large tangled web of numerical complexity due to the vast amount of physical processes involved. Understanding the numerical dependencies that act on the galactic dynamo is a crucial step in determining what resolution and conditions are required to properly capture the magnetic fields observed in galaxies. Here, we present an extensive study on the numerical dependencies of the galactic dynamo in isolated spiral galaxies using smoothed particle magnetohydrodynamics. We performed 53 isolated spiral galaxy simulations with different initial setups, feedback, resolution, Jeans floor, and dissipation parameters. The results show a strong mean-field dynamo occurring in the spiral-arm region of the disk, likely produced by the classical alpha-omega dynamo or the recently described gravitational instability dynamo. The inclusion of feedback is seen to work in both a destructive and positive fashion for the amplification process. Destructive interference for the amplification occurs due to the breakdown of filament structure in the disk, the increase of turbulent diffusion, and the ejection of magnetic flux from the central plane to the circumgalactic medium. The positive effect of feedback is the increase in vertical motions and the turbulent fountain flows that develop, showing a high dependence on the small-scale vertical structure and the numerical dissipation within the galaxy. Galaxies with an effective dynamo saturate their magnetic energy density at levels between 10 and 30% of the thermal energy density. The density-averaged numerical Prandtl number is found to be below unity throughout the galaxy for all our simulations, with an increasing value with radius. Assuming a turbulent injection length of 1 kpc, the numerical magnetic Reynolds number is within the range of Re mag  = 10 − 400, indicating that some regions are below the levels required for the small-scale dynamo (Re mag, crit  = 30 − 2700) to be active.

• Tamfal, Tomas; Mayer, Lucio; Quinn, Thomas R.; Babul, Arif; Madau, Piero & Capelo, Pedro R. [Show all 8 contributors for this article] (2022). The Dawn of Disk Formation in a Milky Way-sized Galaxy Halo: Thin Stellar Disks at z > 4. The Astrophysical Journal (ApJ). ISSN 0004-637X. 928(2). doi: 10.3847/1538-4357/ac558e. Full text in Research Archive Show summary

  We present results from GigaEris, a cosmological, N-body hydrodynamical "zoom-in" simulation of the formation of a Milky Way-sized galaxy halo with unprecedented resolution, encompassing of order a billion particles within the refined region. The simulation employs a modern implementation of smoothed-particle hydrodynamics, including metal-line cooling and metal and thermal diffusion. We focus on the early assembly of the galaxy, down to redshift z = 4.4. The simulated galaxy has properties consistent with extrapolations of the main sequence of star-forming galaxies to higher redshifts and levels off to a star formation rate of ∼60 M⊙ yr−1 at z = 4.4. A compact, thin rotating stellar disk with properties analogous to those of low-redshift systems arises already at z ∼ 8. The galaxy rapidly develops a multi-component structure, and the disk, at least at these early stages, does not grow "upside-down" as often reported in the literature. Rather, at any given time, newly born stars contribute to sustain a thin disk. The kinematics reflect the early, ubiquitous presence of a thin disk, as a stellar disk component with vϕ/σR larger than unity is already present at z ∼ 9–10. Our results suggest that high-resolution spectro-photometric observations of very high-redshift galaxies should find thin rotating disks, consistent with the recent discovery of cold rotating gas disks by ALMA. Finally, we present synthetic images for the James Webb Space Telescope NIRCam camera, showing how the early disk would be easily detectable already at those early times.

• Wissing, Robert; Shen, Sijing; Wadsley, James & Quinn, Thomas (2022). Magnetorotational instability with smoothed particle hydrodynamics. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 659. doi: 10.1051/0004-6361/202141206. Full text in Research Archive Show summary

  The magnetorotational instability (MRI) is an important process in driving turbulence in sufficiently ionized accretion disks. It has been extensively studied using simulations with Eulerian grid codes, but remains fairly unexplored for meshless codes. Here, we present a thorough numerical study on the MRI using the smoothed particle magnetohydrodynamics method with the geometric density average force expression. We performed 37 shearing box simulations with different initial setups and a wide range of resolution and dissipation parameters. We show, for the first time, that MRI with sustained turbulence can be simulated successfully with smoothed-particle hydrodynamics (SPH), with results consistent with prior work with grid-based codes, including saturation properties such as magnetic and kinetic energies and their respective stresses. In particular, for the stratified boxes, our simulations reproduce the characteristic “butterfly” diagram of the MRI dynamo with saturated turbulence for at least 100 orbits. On the contrary, traditional SPH simulations suffer from runaway growth and develop unphysically large azimuthal fields, similar to the results from a recent study with meshless methods. We investigated the dependency of MRI turbulence on the numerical Prandtl number ( P m ) in SPH, focusing on the unstratified, zero net-flux case. We found that turbulence can only be sustained with a Prandtl number larger than ∼2.5, similar to the critical values for the physical Prandtl number found in grid-code simulations. However, unlike grid-based codes, the numerical Prandtl number in SPH increases with resolution, and for a fixed Prandtl number, the resulting magnetic energy and stresses are independent of resolution. Mean-field analyses were performed on all simulations, and the resulting transport coefficients indicate no α -effect in the unstratified cases, but an active αω dynamo and a diamagnetic pumping effect in the stratified medium, which are generally in agreement with previous studies. There is no clear indication of a shear-current dynamo in our simulation, which is likely to be responsible for a weaker mean-field growth in the tall, unstratified, zero net-flux simulation.

• Applebaum, Elaad; Brooks, Alyson M.; Christensen, Charlotte R.; Munshi, Ferah; Quinn, Thomas R. & Shen, Sijing [Show all 7 contributors for this article] (2021). Ultrafaint Dwarfs in a Milky Way Context: Introducing the Mint Condition DC Justice League Simulations. The Astrophysical Journal (ApJ). ISSN 0004-637X. 906(2). doi: 10.3847/1538-4357/abcafa. Full text in Research Archive
• Mina, Mattia; Shen, Sijing; Keller, Benjamin Walter; Mayer, Lucio; Madau, Piero & Wadsley, James (2021). The baryon cycle of Seven Dwarfs with superbubble feedback. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 655. doi: 10.1051/0004-6361/202039420. Full text in Research Archive Show summary

  We present results from a high-resolution, cosmological, ΛCDM simulation of a group of field dwarf galaxies with the “superbubble” model for clustered SN feedback, accounting for thermal conduction and cold gas evaporation. We compared our results to a previous simulation which has the same initial condition and galaxy formation physics (other than SN feedback), but adopts a delayed-cooling model for supernova. The simulated luminous galaxies have blue colors, low star formation efficiencies and metallicities, and high cold gas content, reproducing the observed scaling relations of dwarf galaxies in the Local Volume. Bursty star formation histories and superbubble-driven outflows lead to the formation of kpc-size dark matter (DM) cores when stellar masses reaches M *  &gt; 10 6   M ⊙ , similar to previous findings. However, the superbubble model appears more effective in destroying DM cusps than the delayed-cooling model in the previous study, reflecting a higher coupling efficiency of SN energy with the ISM. On larger scale, superbubble-driven outflows have a more moderate impact: galaxies have higher gas content, more extended stellar discs, and a smaller metal-enriched region in the circumgalactic medium (CGM). The two halos with M vir  ∼ 10 9   M ⊙ , which formed ultra-faint dwarf galaxies with the delayed-cooling mode, remain dark due to the different impact of metal-enriched galactic winds from two nearby luminous galaxies, indicating that the formation of faint dwarfs is highly dependent on feedback and environmental effects. The column density distributions of H  I , Si  II , C  IV and O  VI as a function of the scaled impact parameter ( b / R vir ) are in good agreement with recent observations of CGM around isolated dwarf galaxies. While H  I is ubiquitous with a covering fraction of unity within the CGM, low and intermediate ions like Si  II and C  IV are less extended (typically confined within 0.2 − 0.3  R vir ), and non-detections are common. O  VI is more extended with column density N(O  VI ) ≳ 10 13.5 cm −2 within R vir , but its mass is only 11% of the total CGM oxygen budget, as the diffuse CGM is highly ionised by the UV background. Superbubble feedback produces C  IV and O  VI column densities that are an order of magnitude higher than those in the previous study using delayed-cooling feedback. Thus, the CGM and DM cores are most sensitive probes of feedback mechanisms.

• Gallart, Carme; Monelli, Matteo; Ruiz-Lara, Tomás; Calamida, Annalisa; Cassisi, Santi & Cignoni, M. [Show all 20 contributors for this article] (2021). The Star Formation History of Eridanus II: On the Role of Supernova Feedback in the Quenching of Ultrafaint Dwarf Galaxies. The Astrophysical Journal (ApJ). ISSN 0004-637X. 909(2). doi: 10.3847/1538-4357/abddbe. Full text in Research Archive
• Wissing, Robert & Shen, Sijing (2020). Smoothed particle magnetohydrodynamics with the geometric density average force expression. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 638. doi: 10.1051/0004-6361/201936739. Full text in Research Archive Show summary

  We present a novel method of magnetohydrodynamics (MHD) within the smoothed particle hydrodynamics scheme (SPMHD) using the geometric density average force expression. Geometric density average within smoothed particle hydrodynamics (GDSPH) has recently been shown to reduce the leading order errors and greatly improve the accuracy near density discontinuities, eliminating surface tension effects. Here, we extend the study to investigate how SPMHD benefits from this method. We implement ideal MHD in the GASOLINE2 and CHANGA codes with both GDSPH and traditional smoothed particle hydrodynamics (TSPH) schemes. A constrained hyperbolic divergence cleaning scheme was employed to control the divergence error and a switch for artificial resistivity with minimized dissipation was also used. We tested the codes with a large suite of MHD tests and showed that in all problems, the results are comparable or improved over previous SPMHD implementations. While both GDSPH and TSPH perform well with relatively smooth or highly supersonic flows, GDSPH shows significant improvements in the presence of strong discontinuities and large dynamic scales. In particular, when applied to the astrophysical problem of the collapse of a magnetized cloud, GDSPH realistically captures the development of a magnetic tower and jet launching in the weak-field regime, while exhibiting fast convergence with resolution, whereas TSPH failed to do so. Our new method shows qualitatively similar results to those of the meshless finite mass/volume schemes within the GIZMO code, while remaining computationally less expensive.

• Smith, Matthew C.; Sijacki, Debora & Shen, Sijing (2019). Cosmological simulations of dwarfs: the need for ISM physics beyond SN feedback alone. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 485(3), p. 3317–3333. doi: 10.1093/mnras/stz599. Full text in Research Archive Show summary

  The dominant feedback mechanism in low-mass haloes is usually assumed to take the form of massive stars exploding as supernovae (SNe). We perform very high resolution cosmological zoom-in simulations of five dwarf galaxies to z = 4 with our mechanical SN feedback model. This delivers the correct amount of momentum corresponding to the stage of the SN remnant evolution resolved, and has been shown to lead to realistic dwarf properties in isolated simulations. We find that in four out of our five simulated cosmological dwarfs, SN feedback has insufficient impact resulting in excessive stellar masses, extremely compact sizes and central supersolar stellar metallicities. The failure of the SN feedback in our dwarfs is physical in nature within our model and is the result of the build-up of very dense gas in the early universe due to mergers and cosmic inflows prior to the first SN occurring. We demonstrate that our results are insensitive to resolution (provided that it is high enough), details of the (spatially uniform) UV background and reasonable alterations within our star formation prescription. We therefore conclude that the ability of SNe to regulate dwarf galaxy properties is dependent on other physical processes, such as turbulent pressure support, clustering, and runaway of SN progenitors and other sources of stellar feedback.

• Zana, Tommaso; Capelo, Pedro R.; Dotti, Massimo; Mayer, Lucio; Lupi, Alessandro & Haardt, Francesco [Show all 8 contributors for this article] (2019). Barred galaxies in cosmological zoom-in simulations: the importance of feedback. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 488(2), p. 1864–1877. doi: 10.1093/mnras/stz1834. Full text in Research Archive Show summary

  Bars are a key factor in the long-term evolution of spiral galaxies, in their unique role in redistributing angular momentum and transporting gas and stars on large scales. The Eris-suite simulations are cosmological zoom-in, N-body, smoothed-particle hydrodynamic simulations built to follow the formation and evolution of a Milky Way-sized galaxy across the build-up of the large scale structure. Here we analyse and describe the outcome of two particular simulations taken from the Eris suite – ErisBH and Eris2k – which mainly differ in the prescriptions employed for gas cooling, star formation, and feedback from supernovae and black holes. Our study shows that the enhanced effective feedback in Eris2k, due to the collective effect of the different micro-physics implementations, results in a galaxy which is less massive than its ErisBH counterpart till z ∼ 1. However, when the stellar content is large enough so that global dynamical instabilities can be triggered, the galaxy in Eris2k develops a stronger and more extended bar with respect to ErisBH. We demonstrate that he structural properties and time evolution of the two bars are very different. Our results highlight the importance of accurate sub-grid prescriptions in cosmological zoom-in simulations of the process of galaxy formation and evolution, and the possible use of a statistical sample of barred galaxies to assess the strength of the stellar feedback.

• Smith, Matthew C.; Sijacki, Debora & Shen, Sijing (2018). Supernova feedback in numerical simulations of galaxy formation: Separating physics from numerics. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 478(1), p. 302–331. doi: 10.1093/mnras/sty994. Full text in Research Archive Show summary

  While feedback from massive stars exploding as supernovae (SNe) is thought to be one of the key ingredients regulating galaxy formation, theoretically it is still unclear how the available energy couples to the interstellar medium and how galactic scale outflows are launched. We present a novel implementation of six sub-grid SN feedback schemes in the moving-mesh code AREPO, including injections of thermal and/or kinetic energy, two parametrizations of delayed cooling feedback and a ‘mechanical’ feedback scheme that injects the correct amount of momentum depending on the relevant scale of the SN remnant resolved. All schemes make use of individually time-resolved SN events. Adopting isolated disc galaxy set-ups at different resolutions, with the highest resolution runs reasonably resolving the Sedov–Taylor phase of the SN, we aim to find a physically motivated scheme with as few tunable parameters as possible. As expected, simple injections of energy overcool at all but the highest resolution. Our delayed cooling schemes result in overstrong feedback, destroying the disc. The mechanical feedback scheme is efficient at suppressing star formation, agrees well with the Kennicutt–Schmidt relation, and leads to converged star formation rates and galaxy morphologies with increasing resolution without fine-tuning any parameters. However, we find it difficult to produce outflows with high enough mass loading factors at all but the highest resolution, indicating either that we have oversimplified the evolution of unresolved SN remnants, require other stellar feedback processes to be included, and require a better star formation prescription or most likely some combination of these issues.

• Henden, Nicholas A; Puchwein, Ewald; Shen, Sijing & Sijacki, Debora (2018). The FABLE simulations: A feedback model for galaxies, groups, and clusters. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 479(4), p. 5385–5412. doi: 10.1093/mnras/sty1780. Full text in Research Archive Show summary

  We present the Feedback Acting on Baryons in Large-scale Environments suite of cosmological hydrodynamical simulations of galaxies, groups, and clusters. The simulations use the AREPO moving-mesh code with a set of physical models for galaxy formation based on the successful Illustris simulation, but with updated active galactic nucleus (AGN) and supernovae feedback models. This allows us to simultaneously reproduce the observed redshift evolution of the galaxy stellar mass function together with the stellar and gas mass fractions of local groups and clusters across a wide range of halo masses. Focusing on the properties of groups and clusters, we find very good agreement with a range of observed scaling relations, including the X-ray luminosity–total mass and gas mass relations as well as the total mass–temperature and Sunyaev–Zel’dovich flux–mass relations. Careful comparison of our results with scaling relations based on X-ray hydrostatic masses as opposed to weak-lensing-derived masses reveals some discrepancies, which hint towards a non-negligible X-ray mass bias in observed samples. We further show that radial profiles of density, pressure, and temperature of the simulated intracluster medium are in very good agreement with observations, in particular for r > 0.3 r500. In the innermost regions however we find too large entropy cores, which indicates that a more sophisticated modelling of the physics of AGN feedback may be required to accurately reproduce the observed populations of cool-core and non-cool-core clusters.

• Sokołowska, Aleksandra; Babul, Arif; Mayer, Lucio; Shen, Sijing & Madau, Piero (2018). The complementary roles of feedback and mergers in building the gaseous halo and the X-ray corona of Milky-Way-sized galaxies. The Astrophysical Journal (ApJ). ISSN 0004-637X. 867:73, p. 1–18. doi: 10.3847/1538-4357/aae43a. Full text in Research Archive Show summary

  We use high-resolution cosmological hydrodynamical simulations of Milky-Way-sized galaxies with varying supernova feedback strengths and merger histories to investigate the formation of their gaseous halos and especially their hot (>106 K) X-ray-luminous coronae. Our simulations predict the presence of significant hot gas in the halos as early as z = 3–4, well before the halos ought to be able to sustain hot mode accretion in the conventional picture. The nascent coronae grow inside-out and initially do so primarily as a result of outflows from the central galaxies powered by merger-induced shock heating and strong supernova feedback, both of which are elemental features of today’s successful galaxy formation models. Furthermore, the outflows and the forming coronae also accelerate the transition from cold to hot mode accretion by contributing to the conditions for sustaining stable accretion shocks. They also disrupt the filamentary streams funneling cold gas onto the central galaxies by causing their mouths to fray into a broad delta, detach from the galaxies, and be pushed away to larger radii. And even though at early times the filaments repeatedly re-form, the hot gas and the outflows act to weaken the filaments and accelerate their ultimate disruption. Although galactic outflows are generally thought of as ejective feedback, we find that their action on the filaments suggests a preventive role as well. © 2018 American Astronomical Society

• Christensen, Charlotte R.; Davé, Romeel; Brooks, Alyson M.; Quinn, Thomas & Shen, Sijing (2018). Tracing outflowing metals in simulations of dwarf and spiral galaxies. The Astrophysical Journal (ApJ). ISSN 0004-637X. 867(2). doi: 10.3847/1538-4357/aae374. Full text in Research Archive Show summary

  We analyze the metal accumulation in dwarf and spiral galaxies by following the history of metal enrichment and outflows in a suite of 20 high-resolution simulated galaxies. These simulations agree with the observed stellar and gas-phase mass–metallicity relation, an agreement that relies on large fractions of the produced metals escaping into the circumgalactic media. For instance, in galaxies with Mvir ∼ 109.5–1010 M, we find that about ∼85% of the available metals are outside of the galactic disk at z = 0, although the fraction decreases to a little less than half in Milky-Way-mass galaxies. In many cases, these metals are spread far beyond the virial radius. We analyze the metal deficit within the ISM and stars in the context of previous work tracking the inflow and outflow of baryons. Outflows are prevalent across the entire mass range, as is reaccretion. We find that between 40% and 80% of all metals removed from the galactic disk are later reaccreted. The outflows themselves are metal-enriched relative to the ISM by a factor of 0.2 dex because of the correspondence between sites of metal enrichment and outflows. As a result, the metal mass loading factor scales as v metals circ 0.91 h µ - , a somewhat shallower scaling than the total mass loading factor. We analyze the simulated galaxies within the context of analytic chemical evolution models by determining their net metal expulsion efficiencies, which encapsulate the rates of metal loss and reaccretion. We discuss these results in light of the inflow and outflow properties necessary for reproducing the mass–metallicity relation.

• Zana, Tommaso; Dotti, Massimo; Capelo, Pedro R.; Mayer, Lucio; Haardt, Francesco & Shen, Sijing [Show all 7 contributors for this article] (2018). Bar resilience to flybys in a cosmological framework. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 479(4), p. 5214–5219. doi: 10.1093/mnras/sty1850. Full text in Research Archive Show summary

  It has been proposed that close interactions with satellite galaxies can significantly perturb the morphology of the main galaxy. However, the dynamics of an already formed bar following the interaction with the external environment has not been studied in detail in a fully cosmological context. In this work, analysing the cosmological zoom-in simulation Eris2k, we study the effects that a very unequal-mass flyby crossing the stellar disc has on the stability of the pre-existing bar. We characterize the evolution of the bar strength and length showing that the perturbation exerted by the flyby shuffles the orbits of stars for less than one Gyr. After this time, the bar shows a remarkable resilience, reforming with properties comparable to those it had before the interaction. Our work shows that close unequal-mass encounters, the most frequent interactions occurring during the evolution of cosmic structures, have (i) an overall minor impact on the global evolution of the bar in the long term, still (ii) the effect is destructive, and (iii) a very weak interaction is sufficient to dismantle a strong bar leading to its “apparent death”. As a consequence, due to the non-negligible duration of the bar-less period, a fraction of observed spiral galaxies classified as non-barred could be prone to bar formation.

• Shen, Sijing; Kulkarni, Girish; Madau, Piero & Mayer, Lucio (2017). Chemical enrichment of stars due to accretion from the ISM during the Galaxy's assembly. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 469(4), p. 4012–4021. doi: 10.1093/mnras/stx1094. Full text in Research Archive Show summary

  Using the Eris zoom-in cosmological simulation of assembly of a Milky Way analogue, we study the chemical enrichment of stars due to accretion of metal-enriched gas from the interstellar medium (ISM) during the Galaxy's development. We consider metal-poor and old stars in the Galactic halo and bulge through the use of stellar orbits, gas density and metallicity distributions in Eris. Assuming spherically symmetric Bondi–Hoyle accretion, we find that halo and bulge stars accrete metals at the rate of about 10−24 and 10−22 M⊙ yr−1, respectively, at redshifts z ≲ 3, but this accretion rate increases roughly a hundred-fold to about 10−20 M⊙ yr−1 at higher redshifts due to increased gas density. Bulge and halo stars accrete similar amounts of metals at high redshifts when kinematically distinct bulge and halo have not yet developed, and both sets of stars encounter a similar metal distribution in the ISM. Accretion alone can enrich main-sequence stars up to [Fe/H] ∼ −2 in extreme cases, with the median enrichment level due to accretion of about [Fe/H] ∼ −6 to −5. Because accretion mostly takes place at high redshifts, it is α-enriched to [α/Fe] ∼ 0.5. We find that accretive metal enrichment is sufficient to affect the predicted metallicity distribution function of halo stars at [Fe/H] < −5. This can hinder attempts to infer natal chemical environment of metal-poor stars from their observed enrichment. Peculiar enrichment patterns such as those predicted to arise from pair-instability supernovae could help in disentangling the natal and accreted metal content of stars. © 2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society

• Chakrabarti, Sukanya; Chang, Philip; O'Shaughnessy, Richard; Brooks, Alyson M.; Shen, Sijing & Bellovary, Jillian [Show all 8 contributors for this article] (2017). The Contribution of Outer H I Disks to the Merging Binary Black Hole Population. Astrophysical Journal Letters. ISSN 2041-8205. 850(1), p. 1–5. doi: 10.3847/2041-8213/aa9655. Full text in Research Archive Show summary

  We investigate the contribution of outer H i disks to the observable population of merging black hole binaries. Like dwarf galaxies, the outer H i disks of spirals have low star formation rates and lower metallicities than the inner disks of spirals. Since low-metallicity star formation can produce more detectable compact binaries than typical star formation, the environments in the outskirts of spiral galaxies may be conducive to producing a rich population of massive binary black holes. We consider here both detailed controlled simulations of spirals and cosmological simulations, as well as the current range of observed values for metallicity and star formation in outer disks. We find that the outer H i disks contribute at least as much as dwarf galaxies do to the observed LIGO/Virgo detection rates. Identifying the host galaxies of merging massive black holes should provide constraints on cosmological parameters and insights into the formation channels of binary mergers. © 2017. The American Astronomical Society. All rights reserved.

• Capelo, Pedro R.; Dotti, Massimo; Volonteri, Marta; Mayer, Lucio; Bellovary, Jillian & Shen, Sijing (2017). A survey of dual active galactic nuclei in simulations of galaxy mergers: frequency and properties. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 469(4), p. 4437–4454. doi: 10.1093/mnras/stx1067. Full text in Research Archive
• Sokołowska, Aleksandra; Capelo, Pedro R.; Fall, S. Michael; Mayer, Lucio; Shen, Sijing & Bonoli, Silvia (2017). Galactic Angular Momentum in Cosmological Zoom-in Simulations. I. Disk and Bulge Components and the Galaxy–Halo Connection. The Astrophysical Journal (ApJ). ISSN 0004-637X. 835(2). doi: 10.3847/1538-4357/835/2/289. Full text in Research Archive
• Cole, Andrew A.; Weisz, Daniel R.; Skillman, Evan D.; Leaman, Ryan; Williams, Benjamin F. & Dolphin, Andrew E. [Show all 14 contributors for this article] (2017). DDO 216-A1: A Central Globular Cluster in a Low-luminosity Transition-type Galaxy. The Astrophysical Journal (ApJ). ISSN 0004-637X. 837(1). doi: 10.3847/1538-4357/aa5df6. Full text in Research Archive
• O'Shaughnessy, R.; Bellovary, Jillian; Brooks, A; Shen, Sijing; Governato, Fabio & Christensen, C. R. (2016). The effects of host galaxy properties on merging compact binaries detectable by LIGO. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 464(3), p. 2831–2839. doi: 10.1093/mnras/stw2550. Full text in Research Archive

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• Zimmermann, Tim; Shen, Sijing & Winther, Hans Arnold (2025). On the wave-like nature of fuzzy dark matter. Modeling, phenomenology, and limits from the bottom up. Universitetet i Oslo. Full text in Research Archive Show summary

  Cold dark matter (CDM), a thermally produced, nonrelativistic form of matter, has established itself as a successful effective theory for the abundant nonluminous matter in the universe, accurately describing the large-scale structure of the universe. At the same time, null results of direct detection efforts emphasize that the microphysical nature of DM remains elusive. With hard detection limits approaching and a plethora of phenomenological degeneracies lurking on galactic scales, looking beyond CDM remains a vital effort in constraining the nature of DM. This thesis investigates ultralight or fuzzy dark matter (FDM) as a theoretically well-motivated alternative to CDM and focuses on its astrophysical implications due to its wave-like nature. To this end, we adopt a bottom-up methodology and examine the behavior of FDM dimension-by-dimension, state-by-state, and filament-by-filament. The results of this thesis are (i) a deepened understanding of its numerical modeling, (ii) a new, robust limit for its mass, and (iii) first steps towards a principled understanding of the emerging wave phenomena in cosmic filaments.

• Baumschlager, Bernhard & Shen, Sijing (2025). The Role of Radiation in Galaxy Formation and Evolution. Universitetet i Oslo. Full text in Research Archive Show summary

  My PhD thesis investigates how ionising ultraviolet (UV) radiation from distant galaxies and active galactic nuclei, summarised as UV background radiation, and from locally formed stars impact the evolution of low mass dwarf galaxies (DGs) by means of numerical radiative transfer (RT) simulations. The first part of the thesis goes through the development of a new numerical scheme for RT and the discretisation of the source spectrum. These methods are designed for galactic scale simulations with a strong focus on computational efficiency. The second part compares our RT simulation of a group of DGs to a simulation of the same group without radiative transfer. We find that radiation from the UV background has a significant impact on the rate at which stars are formed within the early universe. At later times radiation form local stars heats the gas within the DGs, this results in a less bursty star formation rate and an overall suppression of the star formation. The total stellar mass between these two simulation runs only differs by a factor of about two to three. These changes in the stellar mass build up subsequently influence many properties of these DGs.

• Schimek, Alice; Cicone, Claudia; Shen, Sijing & Mroczkowski, Tony (2024). Modelling of sub-mm signals from galaxies and their circum-galactic medium for future facilities: Towards the Atacama Large Aperture Submillimeter Telescope (AtLAST). Universitetet i Oslo. Full text in Research Archive Show summary

  My PhD thesis investigates the gas halo around galaxies, known as the circum-galactic medium (CGM), crucial for understanding galaxy evolution but challenging to observe due to its faint, diffuse and extended nature. I utilized cutting-edge simulations to probe this gas across cosmic time and created mock observations for the upcoming Atacama Large Aperture Submillimeter Telescope (AtLAST). Starting with the early Universe, my findings indicate that AtLAST could detect faint emission lines from the CGM that are currently beyond the reach of telescopes like ALMA. I also explored how sub-millimetre emission line ratios can reveal physical properties of the gas in distant galaxies. Additionally, I mentored a Master's student to enhance our models considering early galaxies' chemical variances, yielding more accurate predictions. Lastly, I modelled the thermal Sunyaev-Zel'dovich effect in the CGM, which reflects internal halo pressures linked to galaxy evolution. I discovered AtLAST could detect this effect in individual galaxy groups, unlike current facillities which can only achieve this via stacking of hundreds of thousand observations. My work demonstrates AtLAST's potential to significantly advance our understanding of galaxy evolution from the early Universe to the present day. My work demonstrates AtLAST's potential to significantly advance our understanding of galaxy evolution from the early Universe to the present day.

• Arroyave, Isabel Montoya; Cicone, Claudia; Shen, Sijing & Winther, Hans Arnold (2024). Multi-tracer molecular gas observations of local infrared luminous galaxies. Universitetet i Oslo. Full text in Research Archive Show summary

  In my thesis, I investigated local (ultra-)luminous infrared galaxies, typically identified as galaxy mergers. These galaxies are fascinating due to their intense star formation and active galactic nuclei, which can lead to powerful galactic outflows spreading gas across large distances. My research focused on the molecular gas phase, a critical component for star formation and the evolution of galaxies. Using high-sensitivity observations, I investigated carbon monoxide and atomic carbon emissions to gain insights into the properties of their molecular gas. Special attention was given to the extended and diffuse gas components, which could be linked to galactic outflows. My findings revealed a wide range of gas excitation states, pointing to extreme conditions throughout the interstellar medium of these galaxies. Moreover, I identified a potential connection between high gas excitation and the presence of powerful outflows. My work highlights the importance of sensitive observations for capturing the full picture of these extreme sources, especially in understanding how star formation and galactic nuclei activities impact their evolution.

• Wissing, Robert Bleckert Nils Håkan & Shen, Sijing (2022). Simulating Galactic Dynamo Processes with Smoothed Particle Magnetohydrodynamics. Universitetet i Oslo. Full text in Research Archive Show summary

  When we look out at the universe, we find that magnetic fields are everywhere. From solar flares to the launching of jets, magnetic fields play an important role for a wide range of systems. This includes galaxies, which have been found to host surprisingly strong magnetic fields compared to their predicted strength in the early universe. This means that there must have been significant amplification of the magnetic field during the evolution of these galaxies. But what kind of process could amplify the magnetic field this quickly? Enter magnetic dynamo processes, which describes the exponential growth of magnetic fields due to being stretched, twisted, and folded by the underlying fluid motions. Due to astronomy being an observational science, an important approach to testing astrophysical theories is through the use of numerical simulations. This gives us a ’virtual’ lab to understand how a system will evolve in time. This thesis goes through the development of a new numerical method and its application to the magnetized core-collapse, magneto-rotational instability and galaxy formation. In our simulations of galaxy formation, we find that our new method can capture many of the dynamo processes, recreating the amplification needed to explain our observations.

• Ribas, Lluis Mas & Shen, Sijing (2018). Extended Diffuse Emission as a Probe of Galaxy and Reionization Physics. Faculty of Mathematics and Natural Sciences, University of Oslo. Full text in Research Archive

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