The atomic Hydrogen content of the post-reionization Universe

TL;DR

This paper models atomic hydrogen in the universe using simulations and semi-analytical methods, analyzing its distribution, clustering, and implications for future 21cm intensity mapping experiments.

Contribution

It introduces new fitting functions for HI distribution, explores HI clustering and bias evolution, and assesses shot noise for upcoming 21cm surveys.

Findings

HI bias increases with redshift and is flat on large scales.

Shot-noise levels are consistent with previous studies, not limiting for experiments.

HI poor satellites significantly influence the 1-halo term in the power spectrum.

Abstract

We present a comprehensive analysis of atomic hydrogen (HI) properties using a semi-analytical model of galaxy formation and N-body simulations covering a large cosmological volume at high resolution. We examine the HI mass function and the HI density, characterizing both their redshift evolution and their dependence on hosting halo mass. We analyze the HI content of dark matter haloes in the local Universe and up to redshift $z=5$, discussing the contribution of different galaxy properties. We find that different assembly history plays a crucial role in the scatter of this relation. We propose new fitting functions useful for constructing mock HI maps with HOD techniques. We investigate the HI clustering properties relevant for future $21$~cm Intensity Mapping (IM) experiments, including the HI bias and the shot noise level. The HI bias increases with redshift and it is roughly flat on the largest scales probed. The scale dependency is found at progressively larger scales with increasing redshift, apart from a dip feature at $z=0$. The shot-noise values are consistent with the ones inferred by independent studies, confirming that shot-noise will not be a limiting factor for IM experiments. We detail the contribution from various galaxy properties on the HI power spectrum and their relation to the halo bias. We find that HI poor satellite galaxies play an important role at the scales of the 1-halo term. Finally, we present the $21$~cm signal in redshift space, a fundamental prediction to be tested against data from future radio telescopes such as SKA.