2021A&A...648A..25Z

Aims. Due to the fact that HI mass measurements are not available for large galaxy samples at high redshifts, we apply a photometric estimator of the HI-to-stellar mass ratio (M_HI/M_*), calibrated using a local Universe sample of galaxies, to a sample of galaxies at z∼1 in the DEEP2 survey. We use these HI mass estimates to calculate HI mass functions (HIMFs) and cosmic HI mass densities (Ω_HI) as well as to examine the correlation between star formation rates and HI gas content for galaxies at z∼1.
Methods. We have estimated HI gas masses for ∼7000 galaxies in the DEEP2 survey with redshifts in the range 0.75<z<1.4 and stellar masses M_*≥10¹⁰M_☉ using a combination of the rest-frame ultraviolet-optical colour (NUV-r) and stellar mass density (µ_*) to estimate M_HI/M_*.
Results.It is found that the high-mass end of the high-z HIMF is quite similar to that of the local HIMF. The lower limit of Ω_HI,limit=2.1x10^–4h₇₀^–1, obtained by directly integrating the HI mass of galaxies with M_*≥10¹⁰M_☉, confirms that massive star-forming galaxies do not dominate the neutral gas at z∼1. We study the evolution of the HI-to-stellar mass ratio from z∼1 to today and find a steeper relation between the HI gas mass fraction and stellar mass at higher redshifts. Specifically, galaxies with M_*=10¹¹M_☉ at z∼1 are found to have 3-4 times higher neutral gas fractions than local galaxies, while the increase is as high as 4-12 times at M_*=10¹⁰M_☉. The quantity M_HI/SFR exhibits very large scatter, and the scatter increases from factors of 5-7 at z=0 to factors close to 100 at z=1. This implies that there is no relation between HI gas and star formation in high-redshift galaxies. The HI gas must be linked to cosmological gas accretion processes at high redshifts.