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

Abstract

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 (MHI/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 < z < 1.4 and stellar masses M* ≳ 1010 M⊙ using a combination of the rest-frame ultraviolet-optical colour (NUV − r) and stellar mass density (μ*) to estimate MHI/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 ΩHI,limit = 2.1 × 10−4 h70−1, 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 MHI/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.