Detection of HI in distant galaxies using spectral stacking
  Using the Parkes radio telescope, we study the 21cm neutral hydrogen (HI)
properties of a sample of galaxies with redshifts z<0.13 extracted from the
optical 2dF Galaxy Redshift Survey (2dFGRS). Galaxies at 0.04<z<0.13 are
studied using new Parkes observations of a 42deg^2 field near the South
Galactic Pole (SGP). A spectral stacking analysis of the 3,277 2dFGRS objects
within this field results in a convincing 12sigma detection. For the
low-redshift sample at 0<z<0.04, we use the 15,093 2dFGRS galaxies observed by
the HI Parkes All-Sky Survey (HIPASS) and find a 31sigma stacked detection. We
measure average HI masses of (6.93 +/- 0.17)*10^9 h^{-2} Msun and (1.48 +/-
0.03)*10^9 h^{-2} Msun for the SGP and HIPASS samples, respectively. Accounting
for source confusion and sample bias, we find a cosmic HI mass density of
Omega_HI=(3.19_{-0.59} ^{+0.43})*10^{-4} h^{-1} for the SGP sample and
(2.82_{-0.59}^{+0.30})*10^{-4} h^{-1} for the HIPASS sample. This suggests no
(12 +/- 23%) evolution in the cosmic HI density over the last ~1h^{-1} Gyr. Due
to the very large effective volumes, cosmic variance in our determination of
Omega_HI is considerably lower than previous estimates. Our stacking analysis
reproduces and quantifies the expected trends in the HI mass and mass-to-light
ratio of galaxies with redshift, luminosity and colour.
