2013MNRAS.435.2693R

We measure the neutral atomic hydrogen (Hi) 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 (ΩHi) 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)x10^–3, and at z ∼ 0.2 we find ΩHi = (0.34±0.09)x10^–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 Hi 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 Hi gas content in galaxies during the last 2.4Gyr.