Constraining a halo model for cosmological neutral hydrogen

TL;DR

This paper develops a combined halo model to understand the distribution of neutral hydrogen across different redshifts, integrating multiple observational constraints and using MCMC to refine model parameters.

Contribution

It introduces a novel combined halo model constrained by diverse HI observations and provides a detailed analysis of the HI-halo mass relation evolution.

Findings

Model fits all observables but underpredicts DLA bias at z~2.3.

Indications of tension between HI column density distribution and galaxy HI mass function at z~0.

Provides best-fit parameters and a fitting form for HI concentration evolution.

Abstract

We describe a combined halo model to constrain the distribution of neutral hydrogen (HI) in the post-reionization universe. We combine constraints from the various probes of HI at different redshifts: the low-redshift 21-cm emission line surveys, intensity mapping experiments at intermediate redshifts, and the Damped Lyman-Alpha (DLA) observations at higher redshifts. We use a Markov Chain Monte Carlo (MCMC) approach to combine the observations and place constraints on the free parameters in the model. Our best-fit model involves a relation between neutral hydrogen mass $M_{\rm HI}$ and halo mass $M$ with a non-unit slope, and an upper and a lower cutoff. We find that the model fits all the observables but leads to an underprediction of the bias parameter of DLAs at $z \sim 2.3$. We also find indications of a possible tension between the HI column density distribution and the mass function of HI-selected galaxies at $z\sim 0$. We provide the central values of the parameters of the best-fit model so derived. We also provide a fitting form for the derived evolution of the concentration parameter of HI in dark matter haloes, and discuss the implications for the redshift evolution of the HI-halo mass relation.