A study of the HI gas fractions of galaxies at z ~ 1.
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 (Omega_HI), and to examine the correlation between star formation rate and HI gas content, for galaxies at z ~ 1. We have estimated HI gas masses for ~7,000 galaxies in the DEEP2 survey with redshifts in the range 0.75 10^{10} M_solar, using a combination of the rest-frame ultraviolet-optical colour (NUV-r) and stellar mass density mu_* as a way to estimate M_HI/M_*. It is found that the high mass end of high-z HI mass function (HIMF) is quite similar to that of the local HIMF. The lower limit of Omega_HI,limit = 2.1 * 10^{-4} h_70^{-1}, obtained by directly integrating the HI mass of galaxies with M_* > 10^{10} M_solar, confirms that massive star-forming galaxies do not dominate the neutral gas at z ~ 1. The total Omega_HI at z ~ 1 is estimated as 6.2 * 10^{-4} h_70^{-1}, when the contribution from galaxies with M_* < 10^{10} M_solar is estimated. We study the evolution of the HI mass to stellar mass ratio from z ~ 1 to today and find a steeper relation between HI gas mass fraction and stellar mass at higher redshifts. Specifically, galaxies with M_* = 10^{11} M_solar 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_* = 10^{10} M_solar. We also find a large spread in HI depletion time at fixed stellar mass, indicating a weak correlation between HI gas mass and star formation rate.