Detection of H i in distant galaxies using spectral stacking

Using the Parkes radio telescope, we study the 21 cm neutral hydrogen (H I) properties of a sample of galaxies with redshifts z < 0.13 extracted from the optical Two-Degree-Field Galaxy Redshift Survey (2dFGRS). Galaxies at 0.04 < z < 0.13 are studied using new Parkes observations of a 42 deg 2 field near the South Galactic Pole (SGP). A spectral stacking analysis of the 3277 2dFGRS objects within this field results in a convincing 12 detection. For the low-redshift sample at 0 < z < 0.04, we use the 15 093 2dFGRS galaxies observed by the H I Parkes All-Sky Survey (HIPASS) and find a 31 stacked detection. We measure average H I masses of (6.93 0.17) 10 9 and (1.48 0.03) 10 9 h -2 M for the SGP and HIPASS samples, respectively. Accounting for source confusion and sample bias, we find a cosmic H I mass density of H I = (3.19 +0.43 -0.59 ) 10 -4 h -1 for the SGP sample and (2.82 +0.30 -0.59 ) 10 -4 h -1 for the HIPASS sample. This suggests no (12 23 per cent) evolution in the cosmic H I density over the last 1 h -1 Gyr. Due to the very large effective volumes, cosmic variance in our determination of H I is considerably lower than previous estimates. Our stacking analysis reproduces and quantifies the expected trends in the H I mass and mass-to-light ratio of galaxies with redshift, luminosity and colour.