The GMRT CATz1-COSMOS Survey: H i 21 cm Emission from Star-forming Galaxies at z ≈ 1 in the COSMOS Field

Abstract

We report a 216 hr upgraded Giant Metrewave Radio Telescope (GMRT) Band-4 550−850 MHz observation of the COSMOS field, using 7 GMRT pointings to cover an area of 2.4 sq. deg., with on-source times of 67 hr in the central pointing, and 89 hr distributed across the other six pointings. We stacked, at a spatial resolution of 90 kpc, the H i 21 cm emission signals from 8122 star-forming galaxies with accurate spectroscopic redshifts and lying within the FWHM of the GMRT primary beam at their redshifted H i 21 cm frequencies. We detect the stacked H i 21 cm signal from the 8122 galaxies at 4.2σ significance, obtaining an average H i mass of 〈MHI〉 = (15.5 ± 3.7) × 109 M⊙. This is only the second galaxy population at z ≈ 1 with a measurement of the average H i content; the other being the blue star-forming galaxies in the DEEP2 fields. We also detected the median 1.4 GHz radio continuum emission signal from the 8122 galaxies by stacking the GMRT 680 MHz continuum images, obtaining a weighted-median star formation rate (SFR) of 8.90 ± 0.92 M⊙ yr−1. We measure a characteristic H i-to-stellar mass ratio of 〈MHI〉/〈M⋆〉 = (1.15 ± 0.27) and a characteristic H i depletion timescale of 〈MHI〉/SFR = (1.74 ± 0.45) Gyr for star-forming galaxies at 〈z〉 = 1.081 in COSMOS, both consistent with earlier measurements for the blue, star-forming DEEP2 galaxies at z ≈ 1. Our results support the evidence from the DEEP2 survey that the observed decline in the SFR density at z < 1 is due to insufficient accretion of atomic gas from the intergalactic medium to replenish the gas reservoir consumed in the process of star formation.