Measuring the H i Content of Individual Galaxies Out to the Epoch of Reionization with C ii

International audienceThe 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