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

¹ CAS Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China
e-mail: xtwfn@bao.ac.cn
² Max Planck Institut für Astrophysik, Karl-Schwarzschild-Strasse 1, 85748 Garching, Germany
³ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China
⁴ School of Astronomy and Space Science, Nanjing University, Nanjing 210093, PR China
⁵ Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210093, PR China
⁶ Shanghai Astronomical Observatory, 80 Nandan Road, Shanghai 200030, PR China
⁷ Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, CAS, 80 Nandan Road, Shanghai 200030, PR China

Received: 9 November 2020
Accepted: 8 February 2021

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 (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 (Ω_HI) 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 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 = 2.1 × 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 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.