A study of the H I gas fractions of galaxies at z ∼ 1

Aims. Due to the fact that H I mass measurements are not available for large galaxy samples at high redshifts, we apply a photometric estimator of the H I-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 H I mass estimates to calculate H I mass functions (HIMFs) and cosmic H I mass densities (ΩHI ) as well as to examine the correlation between star formation rates and H I gas content for galaxies at z  ∼ 1.Methods. We have estimated H I gas masses for ∼7000 galaxies in the DEEP2 survey with redshifts in the range 0.75  *  ≳ 1010  M ⊙ 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 , obtained by directly integrating the H I mass of galaxies with M *  ≳ 1010  M ⊙ , confirms that massive star-forming galaxies do not dominate the neutral gas at z  ∼ 1. We study the evolution of the H I-to-stellar mass ratio from z  ∼ 1 to today and find a steeper relation between the H I gas mass fraction and stellar mass at higher redshifts. Specifically, galaxies with M *  = 1011  M ⊙ 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 *  = 1010  M ⊙ . 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 H I gas and star formation in high-redshift galaxies. The H I gas must be linked to cosmological gas accretion processes at high redshifts.