  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-backtime of ~3 Gyr). The sample of galaxies studied were selected
from Halpha-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 coadded the HI 21 cm
emission signal for all the galaxies within the GMRT spectral line data cube.
  From the coadded signal of 121 galaxies, we measure an average atomic
hydrogen gas mass of (2.26 +- 0.90)*10^9 solar masses. We translate this HI
signal into a cosmic density of neutral gas at z = 0.24 of Omega_gas = (0.91 +-
0.42)*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 Lyman-alpha systems around this redshift. We also find that the
correlations between the Halpha luminosity and the radio continuum luminosity
and between the star formation rate and the HI 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 star
formation rate is 3 times higher.
