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

Abstract

We report a ≈400 hr Giant Metrewave Radio Telescope (GMRT) search for H i 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 H i 21 cm emission signals from 2841 blue star-forming galaxies that lie within the FWHM of the GMRT primary beam. This yielded a 5.0 σ detection of the average H i 21 cm signal from the 2841 galaxies at an average redshift 〈 z 〉 ≈ 1.3, only the second detection of H i 21 cm emission at z ≥ 1. We obtain an average H i mass of 〈M HI 〉 = (3.09 ± 0.61) × 10 10 M ⊙ and an H i -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 H i 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 H i 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 H i could then explain the observed decline in the cosmic SFR density at z < 1.