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

TL;DR

This study detects and characterizes the distribution of neutral hydrogen in large-scale galaxy haloes using 21-cm intensity maps, providing new insights into HI content and halo properties at low redshift.

Contribution

It presents the first direct measurement of HI halo profiles from 21-cm maps and constrains halo mass and concentration parameters, demonstrating the feasibility of testing theoretical models.

Findings

Detection of HI halo emission at 12.5σ and 13.5σ significance.

Estimated halo mass around 10^16.1 to 10^16.5 solar masses.

Satellite galaxies appear to be HI-richer than centrals.

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.