2016MNRAS.460.2675R

We present the results of H i spectral stacking analysis of Giant Metrewave Radio Telescope (GMRT) observations targeting the Cosmological Evolution Survey (COSMOS) field. The GMRT data cube contains 474 field galaxies with redshifts known from the zCOSMOS-bright 10 k catalogue. Spectra for the galaxies are co-added and the stacked spectrum allows us to make a ∼3σ measurement of the average H i mass. Using this average H i mass, along with the integral optical B-band luminosity of the galaxies and the luminosity density of the COSMOS field, a volume normalization is applied to obtain the cosmic H i mass density (Ω_H i). We find a cosmic H i mass density of Ω_H i=(0.42±0.16) x 10^-3 at z ∼ 0.37, which is the highest redshift measurement of Ω_H i ever made using H i spectral stacking. The value we obtained for Ω_H i at z ∼ 0.37 is consistent with that measured from large blind 21-cm surveys at z=0, as well as measurements from other H i stacking experiments at lower redshifts. Our measurement, in conjunction with earlier measurements, indicates that there has been no significant evolution of H i gas abundance over the last 4 Gyr. A weighted mean of Ω_H i from all 21-cm measurements at redshifts z <= 0.4 gives Ω_H i=(0.35±0.01) x 10^-3. The Ω_H i measured (from H i 21-cm emission measurements) at z <= 0.4 is, however, approximately half that measured from damped Lyman-α absorption (DLA) systems at z >= 2. Deeper surveys with existing and upcoming instruments will be critical to understand the evolution of Ω_H i in the redshift range intermediate between z ∼ 0.4 and the range probed by DLA observations.