The neutral hydrogen distribution in large-scale haloes from 21-cm intensity maps

Abstract: We detect the neutral hydrogen (HI) radial brightness temperature profile in large-scale haloes by stacking 48,430 galaxies selected from the 2dFGRS catalogue onto a set of 21-cm intensity maps obtained with the Parkes radio telescope, spanning a total area of $\sim$1,300 $\text{deg}^2$ on the sky and covering the redshift range $0.06\lesssim z\lesssim 0.10$. Maps are obtained by removing both 10 and 20 foreground modes in the principal component analysis. We perform the stack at the map level and extract the profile from a circularly symmetrised version of the halo emission. We detect the HI halo emission with the significance $12.5\sigma$ for the 10-mode and $13.5\sigma$ for the 20-mode removed maps at the profile peak. We jointly fit for the observed halo mass $M_{\rm v}$ and the normalisation $c_{0,\rm HI}$ for the HI concentration parameter against the reconstructed profiles, using functional forms for the HI halo abundance proposed in the literature. We find $\log_{10}{(M_{\rm v}/\text{M}{\odot})} = 16.1^{+0.1}{-0.2}$, $c_{0,\rm HI}=3.5^{+0.7}_{-1.0}$ for the 10-mode and $\log_{10}{(M_{\rm v}/\text{M}{\odot})} = 16.5^{+0.1}{-0.2}$, $c_{0,\rm HI}=5.3^{+1.1}_{-1.7}$ for the 20-mode removed maps. These estimates show the detection of the integrated contribution from multiple galaxies located inside very massive haloes. We also consider sub-samples of 13,979 central and 34,361 satellite 2dF galaxies separately, and obtain marginal differences suggesting satellite galaxies are HI-richer. This work shows for the first time the feasibility of testing theoretical models for the HI halo content directly on profiles extracted from 21-cm maps and opens future possibilities for exploiting upcoming HI intensity-mapping data.