The GMRT CAT$z$1-COSMOS Survey: HI 21 cm emission from star-forming galaxies at $z\approx 1$ 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 $67$ hrs in the central pointing, and $89$ hrs distributed across the other six pointings. We stacked, at a spatial resolution of 90 kpc, the HI 21 cm emission signals from 8,122 star-forming galaxies with accurate spectroscopic redshifts and lying within the FWHM of the GMRT primary beam at their redshifted HI 21 cm frequencies. We detect the stacked HI 21 cm signal from the 8,122 galaxies at $4.2\sigma$ significance, obtaining an average HI mass of $⟨M_{\text{HI}}⟩=(15.5\pm 3.7)\times 10^9{\text{M}}_{\odot }$. This is only the second galaxy population at $z\approx 1$ with a measurement of the average HI 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 8,122 galaxies by stacking the GMRT 680 MHz continuum images, obtaining a weighted-median star-formation rate (SFR) of $8.90\pm 0.92M_{\odot }$ yr${}^{-1}$. We measure a characteristic HI-to-stellar mass ratio of $⟨M_{\text{HI}}⟩/⟨M_{\star }⟩=(1.15\pm 0.27)$ and a characteristic HI depletion timescale of $⟨M_{\text{HI}}⟩/\text{SFR}=(1.74\pm 0.45)$ Gyr for star-forming galaxies at $⟨z⟩=1.081$ in COSMOS, both consistent with earlier measurements for the blue, star-forming DEEP2 galaxies at $z\approx 1$. 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.

13 pages, 5 figures, and 2 tables. Accepted for publication in the ApJ Letters. Updated version after correcting a typo