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

Abstract

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 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 HI 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 HI 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 Lyα systems around this redshift. We also find that the correlations between the Hα luminosity and the radio continuum luminosity and between the star formation rate (SFR) 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 SFR is three times higher.