The morphology of the redshifted 21-cm signal from the Cosmic Dawn

TL;DR

This paper analyzes the morphological features of the 21-cm signal during the Cosmic Dawn, revealing how emission and absorption regions evolve and percolate, with complex topologies and filamentary structures, using simulated data and shape diagnostics.

Contribution

It introduces a detailed morphological analysis of 21-cm signal regions during Cosmic Dawn using the SURFGEN2 tool, highlighting percolation thresholds and topological complexity.

Findings

Emission regions percolate at filling factor > 0.15

Absorption regions percolate at filling factor > 0.05

Regions are highly filamentary and topologically complex

Abstract

The spatial fluctuations in the tomographic maps of the redshifted 21-cm signal from the Cosmic Dawn (CD) crucially depend on the size and distribution of the regions with gas temperatures larger than the radio background temperature. In this article, we study the morphological characteristics of such emission regions and their absorption counterparts using the shape diagnostic tool SURFGEN2. Using simulated CD brightness temperature cubes of the 21-cm signal, we find that the emission regions percolate at stages with the filling factor of the emission regions $FF_{\rm emi}\gtrsim 0.15$. Percolation of the absorption regions occurs for $FF_{\rm abs}\gtrsim 0.05$. The largest emission and absorption regions are topologically complex and highly filamentary for most parts of the CD. The number density of these regions as a function of the volume shows the power-law nature with the power-law indexes $\approx -2$ and $-1.6$ for the emission and absorption regions, respectively. Overall, the planarity, filamentarity and genus increase with the increase of the volume of both emission and absorption regions.