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

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 HI signal stacking technique is applied: HI 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 HI mass of galaxies in the two redshift bins. We calculate the cosmic HI gas densities (ω_HI) at the two redshift regimes and compare those with measurements at other redshifts to investigate the global evolution of the HI 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 ω_HI = (0.34 ± 0.09) × 10-3, based on 96 galaxies. These measurements help bridge the gap between high-z damped Lyman a 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 HI gas density at z ~ 0 and that all measurements of ω_HI from 21-cm emission observations at z ω < 0.2 are in agreement with no evolution of the HI gas content in galaxies during the last 2.4Gyr.