Measuring the H I Content of Individual Galaxies Out to the Epoch of Reionization with [C II]

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.