Giant Metrewave Radio Telescope Detection of Hi 21 cm Emission from Star-forming Galaxies at z approximate to 1.3

Abstract

We report a ≈400 hr Giant Metrewave Radio Telescope (GMRT) search for Hi 21 cm emission from star-forming galaxies at z = 1.18–1.39 in seven fields of the DEEP2 Galaxy Survey. Including data from an earlier 60 hr GMRT observing run, we co-added the Hi 21 cm emission signals from 2841 blue star-forming galaxies that lie within the FWHM of the GMRT primary beam. This yielded a 5.0σ detection of the average Hi 21 cm signal from the 2841 galaxies at an average redshift 〈z〉 ≈ 1.3, only the second detection of Hi 21 cm emission at z ≥ 1. We obtain an average Hi mass of 〈MHI〉 = (3.09 ± 0.61) × 1010 M⊙ and an Hi-to-stellar mass ratio of 2.6 ± 0.5, both significantly higher than values in galaxies with similar stellar masses in the local universe. We also stacked the 1.4 GHz continuum emission of the galaxies to obtain a median star formation rate (SFR) of 14.5 ± 1.1 M⊙yr−1. This implies an average Hi depletion timescale of ≈2 Gyr for blue star-forming galaxies at z ≈ 1.3, a factor of ≈3.5 lower than that of similar local galaxies. Our results suggest that the Hi content of galaxies toward the end of the epoch of peak cosmic SFR density is insufficient to sustain their high SFR for more than ≈2 Gyr. Insufficient gas accretion to replenish the Hi could then explain the observed decline in the cosmic SFR density at z < 1.