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

Understand

Using a spectral stacking technique, we measure the neutral hydrogen (HI) properties of a sample of galaxies at $z < 0.11$ across 35 pointings of the Westerbork Synthesis Radio Telescope (WSRT).

• The radio data contains 1,895 galaxies with redshifts and positions known from the Sloan Digital Sky Survey (SDSS).
• 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 HI mass density.
• We find a cosmic HI mass density of $\Omega_{\rm HI} = (4.02 \pm 0.26)\times 10^{-4} h_{70}^{-1}$ at $\langle z\rangle = 0.066$, consistent with measurements from blind HI surveys and other HI stacking experiments at low redshifts.