Investigating the [C ii]-to-H i Conversion Factor and the H i Gas Budget of Galaxies at z ≈ 6 with Hydrodynamic Simulations

Abstract

Abstract One of the most fundamental baryonic matter components of galaxies is the neutral atomic hydrogen (H i ). At low redshifts, this component can be traced directly through the 21 cm transition, but to infer the H 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 ii at 158 μ m as a proxy for H i in a set simulated galaxies at z ≈ 6, following the work by Heintz et al. We select 11,125 star-forming galaxies from the simba simulations, with far-infrared line emissions postprocessed and modeled within the S igame framework. We find a strong connection between C ii and H i , with the relation between this C ii -to-H i relation ( β [C II ] ) 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 at z ≈ 6, and specifically the H i gas mass fraction. We find mean values of M H I / M ⋆ = 1.4 and M H I / M bar,tot = 0.45. These results provide strong evidence for H i being the dominant baryonic matter component by mass in galaxies at z ≈ 6.