HI HOD -- I. The Halo Occupation Distribution of HI Galaxies

TL;DR

This paper introduces a new HI-based Halo Occupation Distribution model that predicts galaxy distribution within dark matter halos based on HI mass, validated with simulations, aiding future galaxy survey analyses.

Contribution

The paper develops a novel HI-HOD model that accurately predicts HI galaxy clustering and distribution within halos, independent of optical galaxy properties.

Findings

HOD parameters can be modeled as functions of HI mass.

Clustering depends on HI galaxy radial distribution within halos.

The model reproduces HI mass functions and clustering simultaneously.

Abstract

The next generation of galaxy surveys will provide more precise measurements of galaxy clustering than have previously been possible. The 21-cm radio signals that are emitted from neutral atomic hydrogen (HI) gas will be detected by large-area radio surveys such as WALLABY and the SKA, and deliver galaxy positions and velocities that can be used to measure galaxy clustering statistics. But, to harness this information to improve our cosmological understanding, and learn about the physics of dark matter and dark energy, we need to accurately model the manner in which galaxies detected in HI trace the underlying matter distribution of the Universe. For this purpose, we develop a new HI-based Halo Occupation Distribution (HOD) model, which makes predictions for the number of galaxies present in dark matter halos conditional on their HI mass. The parameterised HOD model is fit and validated using the Dark Sage semi-analytic model, where we show that the HOD parameters can be modelled by simple linear and quadratic functions of HI mass. However, we also find that the clustering predicted by the HOD depends sensitively on the radial distributions of the HI galaxies within their host dark matter halos, which does not follow the NFW profile in the Dark Sage simulation. As such, this work enables -- for the first time -- a simple prescription for placing galaxies of different HI mass within dark matter halos in a way that is able to reproduce the HI mass-dependent galaxy clustering and HI mass function simultaneously and without requiring knowledge of the optical properties of the galaxies. Further efforts are required to demonstrate that this model can be used to produce large ensembles of mock galaxy catalogues for upcoming surveys.