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We present the results of HI spectral stacking analysis of Giant MetrewaveRadio Telescope (GMRT) observations targeting the COSMOS field. The GMRT datacube contains 474 field galaxies with redshifts known from the zCOSMOS-bright10k catalogue. Spectra for the galaxies are co-added and the stacked spectrumallows us to make a $\sim 3\sigma$ measurement of the average HI mass. Usingthis average HI mass along with the integral optical $B$-band luminosity of thegalaxies and the luminosity density of the COSMOS field, a volume normalisationis applied to obtain the cosmic HI mass density ($\Omega_{\rm HI}$). We find acosmic HI mass density of $\Omega_{\rm HI}$ = (0.42 $\pm$ 0.16) $\times$10$^{-3}$ at $z \sim 0.37$, which is the highest-redshift measurement of$\Omega_{\rm HI}$ ever made using HI spectral stacking. The value we obtainedfor $\Omega_{\rm HI}$ at $z \sim 0.37$ is consistent with that measured fromlarge blind 21-cm surveys at $z = 0$ as well as measurements from other HIstacking experiments at lower redshifts. Our measurement in conjunction withearlier measurements indicates that there has been no significant evolution ofHI gas abundance over the last 4 Gyr. A weighted mean of $\Omega_{\rm HI}$ fromall 21-cm measurements at redshifts $z \lesssim 0.4$ gives $\Omega_{\rm HI}$ =(0.35 $\pm$ 0.01) $\times$ 10$^{-3}$. The $\Omega_{\rm HI}$ measured (from HI21-cm emission measurements) at $z \lesssim 0.4$ is however approximately halfthat measured from Damped Lyman-$\alpha$ Absorption (DLA) systems at $z \gtrsim2$. Deeper surveys with existing and upcoming instruments will be critical tounderstand the evolution of $\Omega_{\rm HI}$ in the redshift rangeintermediate between $z \sim 0.4$ and the range probed by DLA observations.

GMRT observation of neutral atomic hydrogen gas in the COSMOS field at $z \sim 0.37$