Studying 21cm power spectrum with one-point statistics

TL;DR

This paper interprets the evolution of the 21cm power spectrum during cosmic dawn and reionization, highlighting how one-point statistics like skewness reveal key heating processes.

Contribution

It provides a physical interpretation of the 21cm power spectrum evolution and identifies skewness as a crucial observable for detecting X-ray heating onset.

Findings

Peaks and dips in the power spectrum relate to X-ray heating.

Skewness of brightness temperature indicates X-ray heating onset.

Decomposition of fluctuations into density, temperature, and neutral fraction.

Abstract

The redshifted 21cm line signal from neutral hydrogens is a promising tool to probe the cosmic dawn and the epoch of reionization (EoR). Ongoing and future low-frequency radio experiments are expected to detect its fluctuations, especially through the power spectrum. In this paper, we give a physical interpretation of the time evolution of the power spectrum of the 21cm brightness temperature fluctuations, which can be decomposed into dark matter density, spin temperature and neutral fraction of hydrogen fluctuations. From the one-point statistics of the fluctuations, such as variance and skewness, we find that the peaks and dips in the time evolution are deeply related to X-ray heating of the intergalactic gas, which controls the spin temperature. We suggest the skewness of the brightness temperature distribution is a key observable to identify the onset of X-ray heating.