Investigating the [C$\mathrm{II}$]-to-H$\mathrm{I}$ conversion factor and the H$\mathrm{I}$ gas budget of galaxies at $z\approx 6$ with hydrodynamical simulations

Abstract

One of the most fundamental baryonic matter components of galaxies is the neutral atomic hydrogen (H$\mathrm{I}$). At low redshifts, this component can be traced directly through the 21-cm transition, but to infer H$\mathrm{I}$ gas content of the most distant galaxies, a viable tracer is needed. We here investigate the fidelity of the fine structure transition of the (${}^2{P}_{3/2}{-}^2{P}_{1/3}$) transition of singly-ionized carbon [C$\mathrm{II}$] at $158\mu$m as a proxy for H$\mathrm{I}$ in a set simulated galaxies at $z\approx 6$, following the work by Heintz et al. (2021). We select 11,125 star-forming galaxies from the SIMBA simulations, with far-infrared line emissions post-processed and modeled within the SIGAME framework. We find a strong connection between [C$\mathrm{II}$] and H$\mathrm{I}$, with the relation between this [C$\mathrm{II}$]-to-H$\mathrm{I}$ relation (${\beta }_{[\mathrm{C}\mathrm{II}]}$) being anti-correlated 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 at $z\approx 6$, and specifically the HI gas mass fraction. We find mean values of $M_{\mathrm{HI}}/M_{\star }=1.4$, and $M_{\mathrm{HI}}/M_{\mathrm{bar},\mathrm{tot}}=0.45$. These results provide strong evidence for H$\mathrm{I}$ being the dominant baryonic matter component by mass in galaxies at $z\approx 6$.

7 pages, 3 figures. Accepted for publication by ApJL