Atomic Hydrogen in Star-forming Galaxies at Intermediate Redshifts

Abstract

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 (HI) mass and median star formation rate (SFR) of star-forming galaxies, as well as the cosmic HI mass density, at 0.2<z<0.4. This was done by stacking the HI 21cm emission and the rest-frame 1.4 GHz radio continuum from 445 blue star-forming galaxies with MB≤−17 at zmean≈0.34. The stacked HI 21cm emission signal is detected at ≈7σ significance, implying an average HI mass of ⟨MHI⟩=(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 average atomic gas depletion time scale of ⟨ΔtHI⟩≈ 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 HI 21cm emission signal to infer the normalized cosmic HI mass density (ρHI/ρc,0) in star-forming galaxies at z≈0.34. Assuming the local relation between HI mass and absolute B-magnitude, we obtain ρHI/ρc,0=(4.81±0.75)×10^-4, implying no significant evolution in ρHI/ρc,0 from z≈0.4 to the present epoch.