21-cm signatures of residual HI inside cosmic HII regions during reionization

TL;DR

This paper studies how residual neutral hydrogen inside ionized regions affects the 21-cm signal during reionization, revealing that sinks of ionizing radiation dampen signal contrast and complicate detection.

Contribution

It systematically analyzes the impact of sinks of ionizing radiation on 21-cm reionization signals using advanced semi-numerical simulations, highlighting their effects on statistical measures.

Findings

Sinks reduce the peak in the variance during reionization.

Residual HI alters the evolution of skewness and power spectrum.

Large-scale modes offer a clearer signal for probing reionization.

Abstract

We investigate the impact of sinks of ionizing radiation on the reionization-era 21-cm signal, focusing on 1-point statistics. We consider sinks in both the intergalactic medium and inside galaxies. At a fixed filling factor of HII regions, sinks will have two main effects on the 21-cm morphology: (i) as inhomogeneous absorbers of ionizing photons they result in smaller and more widespread cosmic HII patches; and (ii) as reservoirs of neutral gas they contribute a non-zero 21-cm signal in otherwise ionized regions. Both effects damp the contrast between neutral and ionized patches during reionization, making detection of the epoch of reionization with 21-cm interferometry more challenging. Here we systematically investigate these effects using the latest semi-numerical simulations. We find that sinks dramatically suppress the peak in the redshift evolution of the variance, corresponding to the midpoint of reionization. As previously predicted, skewness changes sign at midpoint, but the fluctuations in the residual HI suppress a late-time rise. Furthermore, large levels of residual HI dramatically alter the evolution of the variance, skewness and power spectrum from that seen at lower levels. In general, the evolution of the large-scale modes provides a better, cleaner, higher signal-to-noise probe of reionization.