The spatial distribution of neutral hydrogen as traced by low HI mass galaxies

TL;DR

This study investigates how low HI mass galaxies influence the spatial distribution of neutral hydrogen and its clustering properties, using galaxy formation models to inform 21cm intensity mapping predictions.

Contribution

It provides a detailed analysis of the clustering of low HI mass galaxies and their impact on 21cm signal predictions, highlighting the importance of halo mass resolution.

Findings

Including galaxies with M_HI < 10^8 h^-2 M_sun increases clustering amplitude and slope.

Low HI mass galaxies are typically hosted by haloes >10^12 h^-1 M_sun.

A halo mass resolution of better than 10^10 h^-1 M_sun is needed for accurate 21cm fluctuation predictions.

Abstract

The formation and evolution of galaxies with low neutral atomic hydrogen (HI) masses, M$_{\rm HI}$$<$10$^{8}h^{-2}$M$_{\odot}$, are affected by host dark matter halo mass and photoionisation feedback from the UV background after the end of reionization. We study how the physical processes governing the formation of galaxies with low HI mass are imprinted on the distribution of neutral hydrogen in the Universe using the hierarchical galaxy formation model, GALFORM. We calculate the effect on the correlation function of changing the HI mass detection threshold at redshifts $0 \le z \le 0.5$. We parameterize the clustering as $\xi(r)=(r/r_{0})^{-\gamma}$ and we find that including galaxies with M$_{\rm HI}$$<$10$^{8}h^{-2}$M$_{\odot}$ increases the clustering amplitude $r_{0}$ and slope $\gamma$ compared to samples of higher HI masses. This is due to these galaxies with low HI masses typically being hosted by haloes with masses greater than 10$^{12}{h}^{-1}$M$_{\odot}$, and is in contrast to optically selected surveys for which the inclusion of faint, blue galaxies lowers the clustering amplitude. We show the HI mass function for different host dark matter halo masses and galaxy types (central or satellite) to interpret the values of $r_{0}$ and $\gamma$ of the clustering of HI-selected galaxies. We also predict the contribution of low HI mass galaxies to the 21cm intensity mapping signal. We calculate that a dark matter halo mass resolution better than $\sim$10$^{10}{h}^{-1}$M$_{\odot}$ at redshifts higher than 0.5 is required in order to predict converged 21cm brightness temperature fluctuations.