Using the sample variance of 21 cm maps as a tracer of the ionisation topology

¹ Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France
² Department of Physics, Blackett Laboratory, Imperial College, London SW7 2AZ, UK
³ Department of Physics and McGill Space Institute, McGill University, Montreal H3A 2T8, QC, Canada
e-mail: adelie.gorce@mail.mcgill.ca
⁴ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands

Received: 8 February 2021
Accepted: 28 June 2021

Abstract

Intensity mapping of the 21 cm signal of neutral hydrogen will yield exciting insights into the Epoch of Reionisation and the nature of the first galaxies. However, the large amount of data that will be generated by the next generation of radio telescopes, such as the Square Kilometre Array, as well as the numerous observational obstacles to overcome, require analysis techniques tuned to extract the reionisation history and morphology. In this context, we introduce a one-point statistic, which we refer to as the local variance, σ_loc, that describes the distribution of the mean differential 21 cm brightness temperatures measured in two-dimensional maps along the frequency direction of a light cone. The local variance takes advantage of what is usually considered an observational bias, the sample variance. We find the redshift-evolution of the local variance to not only probe the reionisation history of the observed patches of the sky, but also trace the ionisation morphology. This estimator provides a promising tool to constrain the midpoint of reionisation as well as gain insight into the ionising properties of early galaxies.