The HI content of star-forming galaxies at z=0.24

We use observations from the Giant Metrewave Radio Telescope (GMRT) to measure the atomic hydrogen gas content of star-forming galaxies at z = 0.24 (i.e. a look-back time of similar to 3 Gyr). The sample of galaxies studied were selected from H alpha-emitting field galaxies detected in a narrow-band imaging survey with the Subaru Telescope. The Anglo-Australian Telescope was used to obtain precise optical redshifts for these galaxies. We then co-added the H I 21-cm emission signal for all the galaxies within the GMRT spectral line data cube. From the co-added signal of 121 galaxies, we measure an average atomic hydrogen gas mass of (2.26 +/- 0.90) x 10(9) M-circle dot. We translate this H I signal into a cosmic density of neutral gas at z = 0.24 of Omega(gas) = (0.91 +/- 0.42) x 10(-3). This is the current highest redshift at which Omega(gas) has been constrained from 21-cm emission and our value is consistent with that estimated from damped Ly alpha systems around this redshift. We also find that the correlations between the H alpha luminosity and the radio continuum luminosity and between the star formation rate (SFR) and the H I gas content in star-forming galaxies at z = 0.24 are consistent with the correlations found at z = 0. These two results suggest that the star formation mechanisms in field galaxies similar to 3 Gyr ago were not substantially different from the present, even though the SFR is three times higher.