Presentasjon av masteroppgave: Jessie Harvir Kaur Warraich

The origin of the [CII] emission in high-redshift galaxies

Abstract

The ionized carbon [CII] λ ∼ 158 µm emission line is an important tool in tracing the interstellar medium (ISM) of galaxies, especially at high redshift. The line is one of the dominant coolants of cool gas and, as the line naturally forms in star forming regions, it has also been suggested to be a tracer of the star formation rate (SFR). More and more high-redshift star forming galaxies are being observed in [CII], and robust models of the ISM conditions are therefore crucial for interpreting the data. The goal of this thesis is to investigate the [CII] line emission in high redshift galaxies using a combined theoretical and observational approach.

In the first part of the thesis, I post-process the high-resolution simulation of a Milky-Way type galaxy, “Eris2k”, using the photo-ionization code Cloudy to model the [CII] emission at two redshifts, z ∼ 6 and z ∼ 4.5. Eris2k has a stellar mass of M*=2.8×10^8 M⊙ and 1.2×10^9 M⊙ at z∼6 and z∼4.5 respectively, and a SFR of ∼ 0.51 M⊙ yr^-1 and ∼ 2.9 M⊙ yr^-1 at the same redshifts. The densest (nH>10 cm^-3) and coldest (T<10^4  K) gas particles are treated as individual GMCs as the resolution is m_SPH≃2×10^4 M⊙, while the remaining gas particles are considered as diffuse phase.

Two different models are implemented to estimate the [CII] emission. In the first, simplified model I assume optically thin emission, and heating and ionization from the uniform background radiation of the Milky Way as well as the CMB. In this model, the density and temperature are assumed to be constant within each particle. In a second, more sophisticated “multiphase” model, I differentiate the diffuse phase from the dense phase of the ISM. The diffuse phase is here treated as a complex version of the simplified model, with a background radiation and cosmic rays scaled to the SFR of the simulation and allowing Cloudy to iterate through the cloud until the radiation is absorbed. The dense phase includes molecular and photo-dissociation regions (PDRs). For this denser phase, the density of the gas particle is set to follow a Plummer profile. Furthermore, its treatment includes internal turbulence as well as a variable local radiation field based on nearby stars. To track the contribution from the dense and diffuse gas in the simplified model, the same GMC criteria is applied.

The simplified model estimates total luminosities of L_[CII]=2.68×10^7 L⊙ and 1.74×10^8 L⊙ at z ∼ 6 and 4.5 respectively, from this ∼ 56% and ∼ 64% arises from the dense phase of the ISM. The multiphase model estimates L_[CII]=1.82×10^7 L⊙ and 7.60×10^7 L⊙ at z∼6 and 4.5 respectively, of which ∼ 38% and ∼ 20% are emitted by the dense phase (which is treated separately in this model). For the simplified model, most of the L_[CII] originates from the cold and dense phase with metallicities above 0.1 Z⊙. In contrast, the multiphase model predicts that most of L_[CII] is emitted by the diffuse phase. It is important to note that the luminosity of the diffuse phase is similar in the two models at both redshifts. Instead, the luminosity of the dense phase is ∼ 53% (∼ 83%) lower at z ∼ 6 (4.5) in the multiphase model. I also find that in the multiphase model the [CII] emission extends considerably outside the central ∼ 2 kpc structure, at both redshifts, while the simplified model produces a more compact [CII] source.

Both models predict higher [CII] emission compared to that expected from the local empirical [CII]-SFR relation (De Looze et al., 2014), but the multiphase model is consistent with the upper boundary of the scatter and recent observations.

In the second part of my thesis I study ALMA [CII] observations of two galaxies from the “ALPINE” survey at z ∼ 4.5 (resolution of > 0.7 arcsec). The two galaxies are DC873756 and DC665626 with M*=17.8×10^9  M⊙ and 1.62×10^9  M⊙, and SFR of 5.14 M⊙ yr^-1 and 5.33 M⊙ yr^-1, respectively. Both galaxies have been selected to have a SFR comparable with Eris2k at z ∼ 4.5, while only DC665626 has also a comparable M*. They are both detected in [CII], although DC665626 at much lower S/N. DC873756 shows extended [CII] emission > 10 kpc, which is also seen by the L_[CII] calculations for Eris2k at z ∼ 4.5. DC665626 has similar M*, SFR and L_[CII] as Eris2k at the same redshift, and are therefore comparable to each other. Eris2k shows that galaxies at higher redshifts are likely to be in a turbulent phase of evolution, which could be happening to the observed ones as well. There is nevertheless an interesting resemblance between the observations and the simulated galaxy, however additional multi-wavelength observations are needed to firmly establish the nature of the observed sources.

 

Veiledere: Førsteamanuensis Claudia Cicone og Sijing Shen, Institutt for teoretisk astrofysikk, UiO

Intern sensor: Professor David Fonseca Mota, Institutt for teoretisk astrofysikk, UiO

Ekstern sensor: Doctor (PhD) Alessandro Lupi, University of Milano Bicocca (Università degli Studi di Milano Bicocca)

Publisert 3. juni 2021 14:35 - Sist endret 24. juni 2021 09:54