On the spatial distribution of neutral hydrogen in the Universe: bias and shot-noise of the HI Power Spectrum

TL;DR

This paper develops an analytic framework to understand the distribution, bias, and shot-noise of neutral hydrogen in the Universe, providing insights into its clustering and implications for 21 cm cosmology.

Contribution

It introduces a comprehensive formalism that accounts for all observations at redshift 2.3 and extrapolates to other redshifts, improving understanding of HI bias and shot-noise.

Findings

HI bias is higher than previous estimates.

Shot noise remains subdominant up to redshift 5.

HI power spectrum offers high signal-to-noise ratio in ideal conditions.

Abstract

The spatial distribution of neutral hydrogen (HI) in the Universe contains a wealth of cosmological information. The 21 cm emission line can be used to map the HI up to very high redshift and therefore reveal us something about the evolution of the large scale structures in the Universe. However little is known about the abundance and clustering properties of the HI over cosmic time. Motivated by this, we build an analytic framework where the relevant parameters that govern how the HI is distributed among dark matter halos can be fixed using observations. At the same time we provide tools to study the column density distribution function of the HI absorbers together with their clustering properties. Our formalism is the first one able to account for all observations at a single redshift, $z = 2.3$. The linear bias of the HI and the mean number density of HI sources, two main ingredients in the calculation of the signal-to-noise ratio of a cosmological survey, are then discussed in detail, also extrapolating the results to low and high redshift. We find that HI bias is relatively higher than the value reported in similar studies, but the shot noise level is always sub dominant, making the HI Power Spectrum always a high signal-to-noise measurements up to $z\simeq5$ in the limit of no instrumental noise and foreground contamination.