2021ApJ...922..147H

The H I gas content is a key ingredient in galaxy evolution, the study of which has been limited to moderate cosmological distances for individual galaxies due to the weakness of the hyperfine H I 21 cm transition. Here we present a new approach that allows us to infer the H I gas mass M_HI of individual galaxies up to z ≃ 6, based on a direct measurement of the [C II]-to-H I conversion factor in star-forming galaxies at z >= 2 using γ-ray burst afterglows. By compiling recent [C II]-158 µm emission line measurements we quantify the evolution of the H I content in galaxies through cosmic time. We find that M_HI starts to exceed the stellar mass M_* at z >= 1, and increases as a function of redshift. The H I fraction of the total baryonic mass increases from around 20% at z = 0 to about 60% at z ∼ 6. We further uncover a universal relation between the H I gas fraction M_HI/M_* and the gas-phase metallicity, which seems to hold from z ≃ 6 to z = 0. The majority of galaxies at z > 2 are observed to have H I depletion times, t_dep,HI = M_HI/SFR, less than ≃2 Gyr, substantially shorter than for z ∼ 0 galaxies. Finally, we use the [C II]-to-H I conversion factor to determine the cosmic mass density of H I in galaxies, ρ_HI, at three distinct epochs: z ≃ 0, z ≃ 2, and z ∼ 4-6. These measurements are consistent with previous estimates based on 21 cm H I observations in the local universe and with damped Lyα absorbers (DLAs) at z >= 2, suggesting an overall decrease by a factor of ≃5 in ρ_HI(z) from the end of the reionization epoch to the present.