Fluctuations in 21cm Emission After Reionization

TL;DR

Residual neutral hydrogen in dense regions of the post-reionization intergalactic medium can produce detectable 21cm emission fluctuations, offering insights into the entire reionization process and the state of the IGM.

Contribution

This paper demonstrates that post-reionization 21cm fluctuations are significant and detectable, challenging the assumption that the 21cm signal disappears after reionization.

Findings

Post-reionization 21cm fluctuations are detectable by upcoming observatories.

Residual neutral hydrogen contributes to the 21cm signal after reionization.

Fluctuation statistics probe the entire reionization history.

Abstract

The fluctuations in the emission of redshifted 21cm photons from neutral inter-galactic hydrogen will provide an unprecedented probe of the reionization era. Conventional wisdom assumes that this 21cm signal disappears as soon as reionization is complete, when little atomic hydrogen is left through most of the volume of the inter-galactic medium (IGM). However observations of damped Ly-alpha absorbers indicate that the fraction of hydrogen in its neutral form is significant by mass at all redshifts. Here we use a physically-motivated model to show that residual neutral gas, confined to dense regions in the IGM with a high recombination rate, will generate a significant post-reionization 21cm signal. We show that the power-spectrum of fluctuations in this signal will be detectable by the first generation of low-frequency observatories at a signal-to-noise that is comparable to that achievable in observations of the reionization era. The statistics of 21cm fluctuations will therefore probe not only the pre-reionization IGM, but rather the entire process of HII region overlap, as well as the appearance of the diffuse ionized IGM.