The gas mass of star-forming galaxies at $z \approx 1.3$
  We report a Giant Metrewave Radio Telescope (GMRT) search for HI 21cm
emission from a large sample of star-forming galaxies at $z \approx 1.18 -
1.34$, lying in sub-fields of the DEEP2 Redshift Survey. The search was carried
out by co-adding ("stacking") the HI 21cm emission spectra of 857 galaxies,
after shifting each galaxy's HI 21cm spectrum to its rest frame. We obtain the
$3\sigma$ upper limit S$_{\rm{HI}} < 2.5 \mu$Jy on the average HI 21cm flux
density of the 857 galaxies, at a velocity resolution of $\approx 315$ km
s$^{-1}$. This yields the $3\sigma$ constraint M$_{\rm{HI}} < 2.1 \times
10^{10} \times \left[\Delta {\rm V}/315 \rm{km/s} \right]^{1/2}
\textrm{M}_\odot$ on the average HI mass of the 857 stacked galaxies, the first
direct constraint on the atomic gas mass of galaxies at $z > 1$. The implied
limit on the average atomic gas mass fraction (relative to stars) is ${\rm
M}_{\rm GAS}/{\rm M}_* < 0.5$, comparable to the cold molecular gas mass
fraction in similar star-forming galaxies at these redshifts. We find that the
cosmological mass density of neutral atomic gas in massive star-forming
galaxies at $z \approx 1.3$ is $\Omega_{\rm GAS} < 3.7 \times 10^{-4}$,
significantly lower than $\Omega_{\rm GAS}$ estimates in both galaxies in the
local Universe and damped Lyman-$\alpha$ absorbers at $z \geq 2.2$. Massive
blue star-forming galaxies thus do not appear to dominate the neutral atomic
gas content of the Universe at $z \approx 1.3$.
