Papers › Measuring the HI content of individual galaxies out to the epoch of reionization with [CII]
Measuring the HI content of individual galaxies out to the epoch of reionization with [CII]
Kasper E. Heintz, Darach Watson, Pascal Oesch, Desika Narayanan, Suzanne C. Madden
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The HI 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 HI 21-cm transition. Here we present a new approach that allows us to infer the HI gas mass M_(HI) of individual galaxies up to z≈6, based on a direct measurement of the [CII]-to-HI conversion factor in star-forming galaxies at z≳2 using γ-ray burst afterglows. By compiling recent [CII]-158 μm emission line measurements we quantify the evolution of the HI content in galaxies through cosmic time. We find that the HI mass starts to exceed the stellar mass M_⋆ at z≳1, and increases as a function of redshift. The HI 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 HI 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 HI depletion times, t_(dep,HI) = M_(HI)/SFR, less than ≈2 Gyr, substantially shorter than for z∼0 galaxies. Finally, we use the [CII]-to-HI conversion factor to determine the cosmic mass density of HI 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 HI observations in the local Universe and with damped Lyman-α 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.
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