2019ApJ...882L...7B

We have used the upgraded Giant Metrewave Radio Telescope to carry out a deep (117 on-source hours) L-band observation of the Extended Groth Strip, to measure the average neutral hydrogen (H I) mass and median star formation rate (SFR) of star-forming galaxies, as well as the cosmic H I mass density, at 0.2 < z < 0.4. This was done by stacking the H I 21 cm emission and the rest-frame 1.4 GHz radio continuum from 445 blue star-forming galaxies with M_B <= -17 at z_mean ≃ 0.34. The stacked H I 21 cm emission signal is detected at ≃7σ significance, implying an average H I mass of <M_HI >=(4.93±0.70)×10⁹M_☉. We also stacked the rest-frame 1.4 GHz radio continuum emission of the same galaxies to obtain a median SFR of (0.54 ± 0.06) M_☉ yr^–1; this implies an atomic gas depletion timescale of Δt_H I_ = (≃9) Gyr, consistent with values in star-forming galaxies in the local universe. This indicates that the star formation efficiency does not change significantly over the redshift range 0-0.4. We used the detection of the stacked H I 21 cm emission signal to infer the normalized cosmic H I mass density (ρ_H I_/ρc,0_) in star-forming galaxies at z ≃ 0.34. Assuming the local relation between H I mass and absolute B-magnitude, we obtain ρ_H I_/ρc,0_ = (4.81 ± 0.75) x 10^–4, implying no significant evolution in ρ_H I_/ρc,0_ from z ≃ 0.4 to the present epoch.