The Difference PDF of 21-cm Fluctuations: A Powerful Statistical Tool for Probing Cosmic Reionization

TL;DR

This paper introduces the difference PDF of 21-cm brightness temperature fluctuations as a new statistical tool to better analyze the non-Gaussian features of cosmic reionization, enabling more detailed insights into ionized regions.

Contribution

It proposes and calculates the difference PDF as a novel statistical measure to distinguish ionization fluctuations from density effects during reionization.

Findings

Difference PDF captures non-Gaussian features of 21-cm fluctuations.

It enables separate measurement of ionized region properties.

Provides a more comprehensive analysis than the correlation function.

Abstract

A new generation of radio telescopes are currently being built with the goal of tracing the cosmic distribution of atomic hydrogen at redshifts 6-15 through its 21-cm line. The observations will probe the large-scale brightness fluctuations sourced by ionization fluctuations during cosmic reionization. Since detailed maps will be difficult to extract due to noise and foreground emission, efforts have focused on a statistical detection of the 21-cm fluctuations. During cosmic reionization, these fluctuations are highly non-Gaussian and thus more information can be extracted than just the one-dimensional function that is usually considered, i.e., the correlation function. We calculate a two-dimensional function that if measured observationally would allow a more thorough investigation of the properties of the underlying ionizing sources. This function is the probability distribution function (PDF) of the difference in the 21-cm brightness temperature between two points, as a function of the separation between the points. While the standard correlation function is determined by a complicated mixture of contributions from density and ionization fluctuations, we show that the difference PDF holds the key to separately measuring the statistical properties of the ionized regions.