Giant Metrewave Radio Telescope Detection of HI 21 cm Emission from
  Star-forming Galaxies at $z \approx 1.3$
  We report a $\approx 400$-hour Giant Metrewave Radio Telescope (GMRT) search
for HI 21 cm emission from star-forming galaxies at $z = 1.18-1.39$ in seven
fields of the DEEP2 Galaxy Survey. Including data from an earlier 60-hour GMRT
observing run, we co-added the HI 21 cm emission signals from 2,841 blue
star-forming galaxies that lie within the full-width at half-maximum of the
GMRT primary beam. This yielded a $5.0\sigma$ detection of the average HI 21 cm
signal from the 2,841 galaxies at an average redshift $\langle z \rangle
\approx 1.3$, only the second detection of HI 21 cm emission at $z\ge1$. We
obtain an average HI mass of $\langle {\rm M_{HI}} \rangle=(3.09 \pm 0.61)
\times 10^{10}\ {\rm M}_\odot$ and an HI-to-stellar mass ratio of $2.6\pm0.5$,
both significantly higher than values in galaxies with similar stellar masses
in the local Universe. We also stacked the 1.4 GHz continuum emission of the
galaxies to obtain a median star-formation rate (SFR) of $14.5\pm1.1\ {\rm
M}_\odot \textrm{yr}^{-1}$. This implies an average HI depletion timescale of
$\approx 2$ Gyr for blue star-forming galaxies at $z\approx 1.3$, a factor of
$\approx 3.5$ lower than that of similar local galaxies. Our results suggest
that the HI content of galaxies towards the end of the epoch of peak cosmic SFR
density is insufficient to sustain their high SFR for more than $\approx 2$
Gyr. Insufficient gas accretion to replenish the HI could then explain the
observed decline in the cosmic SFR density at $z< 1$.
