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Abstract One of the most fundamental baryonic matter components of galaxies is the neutral atomic hydrogen (Hi). At low redshifts, this component can be traced directly through the 21 cm transition, but to infer the Higas content of the most distant galaxies, a viable tracer is needed. We here investigate the fidelity of the fine-structure transition of the (²P_3/2−²P_1/3) transition of singly ionized carbon Ciiat 158μm as a proxy for Hiin a set simulated galaxies atz≈ 6, following the work by Heintz et al. We select 11,125 star-forming galaxies from thesimbasimulations, with far-infrared line emissions postprocessed and modeled within the Sigameframework. We find a strong connection between Ciiand Hi, with the relation between this Cii-to-Hirelation (β_[CII]) being anticorrelated with the gas-phase metallicity of the simulated galaxies. We further use these simulations to make predictions for the total baryonic matter content of galaxies atz≈ 6, and specifically the Higas mass fraction. We find mean values ofM_{H I}/M_⋆= 1.4 andM_{H I}/M_bar,tot= 0.45. These results provide strong evidence for Hibeing the dominant baryonic matter component by mass in galaxies atz≈ 6. 

Abstract We present an update to the framework called Simulator of Galaxy Millimeter/submillimeter Emission ( sígame ). sígame derives line emission in the far-infrared (FIR) for galaxies in particle-based cosmological hydrodynamics simulations by applying radiative transfer and physics recipes via a postprocessing step after completion of the simulation. In this version, a new technique is developed to model higher gas densities by parameterizing the probability distribution function (PDF) of the gas density in higher-resolution simulations run with the pseudo-Lagrangian, Voronoi mesh code arepo . The parameterized PDFs are used as a look-up table, and reach higher densities than in previous work. sígame v3 is tested on redshift z = 0 galaxies drawn from the simba cosmological simulation for eight FIR emission lines tracing vastly different phases of the interstellar medium. This version of sígame includes dust radiative transfer with S kirt and high-resolution photoionization models with C loudy , the latter sampled according to the density PDF of the arepo simulations to augment the densities in the cosmological simulation. The quartile distributions of the predicted line luminosities overlap with the observed range for nearby galaxies of similar star formation rate (SFR) for all but two emission lines: [O i ]63 and CO(3–2), which are overestimated by median factors of 1.3 and 1.0 dex, respectively, compared to the observed line–SFR relation of mixed-type galaxies. We attribute the remaining disagreement with observations to the lack of precise attenuation of the interstellar light on sub-grid scales (≲200 pc) and differences in sample selection. 

Modeling emission lines from the millimeter to the UV and producing synthetic spectra is crucial for a good understanding of observations, yet it is an art filled with hazards. This is the proceedings of “Walking the Line”, a 3-day conference held in 2018 that brought together scientists working on different aspects of emission line simulations, in order to share knowledge and discuss the methodology. Emission lines across the spectrum from the millimeter to the UV were discussed, with most of the focus on the interstellar medium, but also some topics on the circumgalactic medium. The most important quality of a useful model is a good synergy with observations and experiments. Challenges in simulating line emission are identified, some of which are already being worked upon, and others that must be addressed in the future for models to agree with observations. Recent advances in several areas aiming at achieving that synergy are summarized here, from micro-physical to galactic and circum-galactic scale. 

Of the almost 40 star-forming galaxies at z≳ 5 (not counting quasi-stellar objects) observed in [{{C}} {{II}}] to date, nearly half are either very faint in [{{C}} {{II}}] or not detected at all, and fall well below expectations based on locally derived relations between star formation rate and [{{C}} {{II}}] luminosity. This has raised questions as to how reliable [{{C}} {{II}}] is as a tracer of star formation activity at these epochs and how factors such as metallicity might affect the [{{C}} {{II}}] emission. Combining cosmological zoom simulations of galaxies with SÍGAME (SImulator of GAlaxy Millimeter/submillimeter Emission), we modeled the multiphased interstellar medium (ISM) and its emission in [{{C}} {{II}}], as well as in [O I] and [O III], from 30 main-sequence galaxies at z≃ 6 with star formation rates ˜3-23 {M}⊙ {yr}}-1, stellar masses ˜ (0.7{--}8)× {10}9 {M}⊙ , and metallicities ˜ (0.1{--}0.4)× {Z}⊙ . The simulations are able to reproduce the aforementioned [{{C}} {{II}}] faintness of some normal star-forming galaxy sources at z≥slant 5. In terms of [O I] and [O III], very few observations are available at z≳ 5, but our simulations match two of the three existing z≳ 5 detections of [O III] and are furthermore roughly consistent with the [O I] and [O III] luminosity relations with star formation rate observed for local starburst galaxies. We find that the [{{C}} {{II}}] emission is dominated by the diffuse ionized gas phase and molecular clouds, which on average contribute ˜66% and ˜27%, respectively. The molecular gas, which constitutes only ˜ 10 % of the total gas mass, is thus a more efficient emitter of [{{C}} {{II}}] than the ionized gas, which makes up ˜85% of the total gas mass. A principal component analysis shows that the [{{C}} {{II}}] luminosity correlates with the star formation activity of a galaxy as well as its average metallicity. The low metallicities of our simulations together with their low molecular gas mass fractions can account for their [{{C}} {{II}}] faintness, and we suggest that these factors may also be responsible for the [{{C}} {{II}}]-faint normal galaxies observed at these early epochs.