HI 21-centimetre emission from an ensemble of galaxies at an average
  redshift of one
  The baryonic processes in galaxy evolution include gas infall onto galaxies
to form neutral atomic hydrogen (HI), the conversion of HI to the molecular
state (H$_2$), and, finally, the conversion of H$_2$ to stars. Understanding
galaxy evolution thus requires understanding the evolution of both the stars,
and the neutral atomic and molecular gas, the primary fuel for star-formation,
in galaxies. For the stars, the cosmic star-formation rate density is known to
peak in the redshift range $z \approx 1-3$, and to decline by an order of
magnitude over the next $\approx 10$ billion years; the causes of this decline
are not known. For the gas, the weakness of the hyperfine HI 21cm transition,
the main tracer of the HI content of galaxies, has meant that it has not
hitherto been possible to measure the atomic gas mass of galaxies at redshifts
higher than $\approx 0.4$; this is a critical lacuna in our understanding of
galaxy evolution. Here, we report a measurement of the average HI mass of
star-forming galaxies at a redshift $z \approx 1$, by stacking their individual
HI 21 cm emission signals. We obtain an average HI mass similar to the average
stellar mass of the sample. We also estimate the average star-formation rate of
the same galaxies from the 1.4 GHz radio continuum, and find that the HI mass
can fuel the observed star-formation rates for only $\approx 1-2$ billion years
in the absence of fresh gas infall. This suggests that gas accretion onto
galaxies at $z < 1$ may have been insufficient to sustain high star-formation
rates in star-forming galaxies. This is likely to be the cause of the decline
in the cosmic star-formation rate density at redshifts below 1.
