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 ${\mathrm{M}}_{\mathrm{B}}\le -17$ at $z_{\mathrm{mean}}\approx 0.34$. The stacked HI 21cm emission signal is detected at $\approx 7\sigma$ significance, implying an average HI mass of $⟨{\mathrm{M}}_{\mathrm{HI}}⟩=(4.93\pm 0.70)\times 10^9{\mathrm{M}}_{\odot }$. We also stacked the rest-frame 1.4 GHz radio continuum emission of the same galaxies, to obtain a median SFR of $(0.54\pm 0.06){\mathrm{M}}_{\odot }$ yr${}^{-1}$; this implies an average atomic gas depletion time scale of $⟨\mathrm{\Delta }{\mathrm{t}}_{\mathrm{HI}}⟩\approx$ 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 $({\rho }_{\mathrm{HI}}/{\rho }_{\mathrm{c},0})$ in star-forming galaxies at $z\approx 0.34$. Assuming the local relation between HI mass and absolute B-magnitude, we obtain ${\rho }_{\mathrm{HI}}/{\rho }_{\mathrm{c},0}=(4.81\pm 0.75)\times 10^{-4}$, implying no significant evolution in ${\rho }_{\mathrm{HI}}/{\rho }_{\mathrm{c},0}$ from $z\approx 0.4$ to the present epoch.

7 pages, 4 figures