Constraining a halo model for cosmological neutral hydrogen

Abstract

We describe a combined halo model to constrain the distribution of neutral hydrogen (H I) in the post-reionization universe. We combine constraints from the various probes of H I 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 approach to combine the observations and place constraints on the free parameters in the model. Our best-fitting model involves a relation between neutral hydrogen mass M H I 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 2.3. We also find indications of a possible tension between the H I column density distribution and the mass function of H I-selected galaxies at z 0. We provide the central values of the parameters of the best-fitting model so derived. We also provide a fitting form for the derived evolution of the concentration parameter of H I in dark matter haloes, and discuss the implications for the redshift evolution of the H I-halo mass relation.