An accurate low-redshift measurement of the cosmic neutral hydrogen density

ABSTRACT Using a spectral stacking technique, we measure the neutral hydrogen (H i) properties of a sample of galaxies at z < 0.11 across 35 pointings of the Westerbork Synthesis Radio Telescope. The radio data contain 1895 galaxies with redshifts and positions known from the Sloan Digital Sky Survey. We carefully quantified the effects of sample bias, aperture used to extract spectra, sidelobes and weighting technique and use our data to provide a new estimate for the cosmic H i mass density. We find a cosmic H i mass density of $\Omega _{\rm H\,{\small I}} = (4.02 \pm 0.26)\times 10^{-4} h_{70}^{-1}$ at 〈z〉 = 0.066, consistent with measurements from blind H i surveys and other H i stacking experiments at low redshifts. The combination of the small interferometer beam size and the large survey volume makes our result highly robust against systematic effects due to confusion at small scales and cosmic variance at large scales. Splitting into three sub-samples with 〈z〉 = 0.038, 0.067, and 0.093 shows no significant evolution of the H i gas content at low redshift.