The 21cm Power Spectrum After Reionization

TL;DR

This paper analyzes the 21cm power spectrum after reionization, showing its close relation to mass density fluctuations and its potential as a precise cosmological probe with upcoming low-frequency radio arrays.

Contribution

It derives the form of ionization modulation on the 21cm power spectrum post-reionization and demonstrates its small impact, simplifying cosmological measurements.

Findings

Modulation of 21cm PS after reionization is less than 1%.

21cm PS post-reionization is primarily related to mass density fluctuations.

Next-generation arrays can measure angular distortions at percent accuracy.

Abstract

We discuss the 21cm power spectrum (PS) following the completion of reionization. In contrast to the reionization era, this PS is proportional to the PS of mass density fluctuations, with only a small modulation due to fluctuations in the ionization field on scales larger than the mean-free-path of ionizing photons. We derive the form of this modulation, and demonstrate that its effect on the 21cm PS will be smaller than 1% for physically plausible models of damped Ly-alpha systems. In contrast to the 21cm PS observed prior to reionization, in which HII regions dominate the ionization structure, the simplicity of the 21cm PS after reionization will enhance its utility as a cosmological probe by removing the need to separate the PS into physical and astrophysical components. As a demonstration, we consider the Alcock-Paczynski test and show that the next generation of low-frequency arrays could measure the angular distortion of the PS at the percent level for z~3-5.