Neutral atomic hydrogen (H i) gas evolution in field galaxies at z ∼ 0.1 and ∼0.2

Abstract

We measure the neutral atomic hydrogen (H i) gas content of field galaxies at intermediate redshifts of z ∼ 0.1 and ∼0.2 using hydrogen 21-cm emission lines observed with the Westerbork Synthesis Radio Telescope. In order to make high signal-to-noise ratio detections, an H i signal stacking technique is applied: H i emission spectra from multiple galaxies, optically selected by the second Canadian Network for Observational Cosmology redshift survey project, are co-added to measure the average H i mass of galaxies in the two redshift bins. We calculate the cosmic H i gas densities (ΩHi) at the two redshift regimes and compare those with measurements at other redshifts to investigate the global evolution of the H i gas density over cosmic time. From a total of 59 galaxies at z ∼ 0.1 we find ΩHi = (0.33 ± 0.05) × 10−3, and at z ∼ 0.2 we find ΩHi = (0.34 ± 0.09) × 10−3, based on 96 galaxies. These measurements help bridge the gap between high-z damped Lyman α observations and blind 21-cm surveys at z = 0. We find that our measurements of ΩHi at z ∼ 0.1 and ∼0.2 are consistent with the H i gas density at z ∼ 0 and that all measurements of ΩHi from 21-cm emission observations at z ≲ 0.2 are in agreement with no evolution of the H i gas content in galaxies during the last 2.4 Gyr.