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We measure the neutral atomic hydrogen (HI) gas content of field galaxies atintermediate redshifts of z ~ 0.1 and z ~ 0.2 using hydrogen 21-cm emissionlines observed with the Westerbork Synthesis Radio Telescope (WSRT). In orderto make high signal-to-noise ratio detections, an HI signal stacking techniqueis applied: HI emission spectra from multiple galaxies, optically selected bythe CNOC2 redshift survey project, are co-added to measure the average HI massof galaxies in the two redshift bins. We calculate the cosmic HI gas densities({\Omega}_{HI}) at the two redshift regimes and compare those with measurementsat other redshifts to investigate the global evolution of the HI gas densityover 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. Thesemeasurements help bridge the gap between high-z damped Lyman-$\alpha$observations and blind 21-cm surveys at $z=$ 0. We find that our measurementsof {\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 observationsat $z \la$ ~ 0.2 are in agreement with no evolution of the HI gas content ingalaxies during the last 2.4 Gyr.

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