The H i 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 ∼3 Gyr). The sample of galaxies studied were selected from Hα-emitting fieldgalaxies 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) × 109 M_⊙. We translate this H i signal into a cosmic density of neutral gas at z = 0.24 of Ω_gas = (0.91 ± 0.42) × 10^-3. This is the current highest redshift at which Ω_gas has been constrained from 21-cm emission and our value is consistent with that estimated from damped Lγα systems around this redshift. We also find that the correlations between the Ha 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 ∼3 Gyr ago were not substantially different from the present, even though the SFR is three times higher.