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