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

We measure the neutral atomic hydrogen (HI) gas content of field galaxies at intermediate redshifts of z ~ 0.1 and z ~ 0.2 using hydrogen 21-cm emission lines observed with the Westerbork Synthesis Radio Telescope (WSRT). 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 CNOC2 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 ({\Omega}_{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 {\Omega}_{HI} = (0.33 $\pm$ 0.05) ~ $\times$ 10$^{-3}$, and at z ~ 0.2 we find {\Omega}_{HI} = (0.34 $\pm$ 0.09) ~ $\times$ 10$^{-3}$, based on 96 galaxies. These measurements help bridge the gap between high-z damped Lyman-$\alpha$ observations and blind 21-cm surveys at $z=$ 0. We find that our measurements of {\Omega}_{HI} at z ~ 0.1 and 0.2 are consistent with the HI gas density at z ~ 0 and that all measurements of {\Omega}_{HI} from 21-cm emission observations at $z \la$ ~ 0.2 are in agreement with no evolution of the HI gas content in galaxies during the last 2.4 Gyr.