2016ApJ...818L..28K

We report a Giant Metrewave Radio Telescope search for H i 21 cm emission from a large sample of massive star-forming galaxies at z ≈ 1.18-1.34, lying in sub-fields of the DEEP2 Redshift Survey. The search was carried out by co-adding (''stacking'') the H i 21 cm emission spectra of 857 galaxies, after shifting each galaxy's H i 21 cm spectrum to its rest frame. We obtain the 3σ upper limit S_ H i_< 2.5 µJy on the average H i 21 cm flux density of the 857 galaxies, at a velocity resolution of ≈315 km/s. This yields the 3σ constraint M_HI < 2.1 x 10¹⁰ x [δV/315 km/s]^1/2M_☉ on the average H i mass of the 857 stacked galaxies, the first direct constraint on the atomic gas mass of galaxies at z > 1. The implied limit on the average atomic gas mass fraction (relative to stars) is M_GAS/M_*< 0.5, comparable to the cold molecular gas mass fraction in similar star-forming galaxies at these redshifts. We find that the cosmological mass density of neutral atomic gas in massive star-forming galaxies at z ≈ 1.3 is Ω_GAS< 3.7x10^–4, significantly lower than Ω_GAS estimates in both galaxies in the local universe and damped Lyα absorbers at z ≥ 2.2. Massive blue star-forming galaxies thus do not appear to dominate the neutral atomic gas content of the universe at z ≈ 1.3.