Opening Reionization: Quantitative Morphology of the Epoch of Reionization and Its Connection to the Cosmic Density Field

TL;DR

This paper introduces a new morphological analysis method for ionized regions during cosmic reionization, revealing detailed insights into bubble sizes, clustering, and percolation thresholds in simulations.

Contribution

It presents a novel spatially resolved morphological characterization technique based on mathematical morphology, applied to reionization simulations to analyze ionized region structures.

Findings

Ionized regions have an average radius of ~8 h^{-1} cMpc at mid-reionization.

Percolation occurs when 25% of the universe is ionized, forming an infinite cluster.

Large ionized regions can have bias values up to 30, indicating strong clustering.

Abstract

We introduce a versatile and spatially resolved morphological characterisation of binary fields, rooted in the opening transform of mathematical morphology. We subsequently apply it to the thresholded ionization field in simulations of cosmic reionization and study the morphology of ionized regions. We find that an ionized volume element typically resides in an ionized region with radius $\sim8\,h^{-1}\mathrm{cMpc}$ at the midpoint of reionization ($z\approx7.5$) and follow the bubble size distribution even beyond the overlap phase. We find that percolation of the fully ionized component sets in when 25% of the universe is ionized and that the resulting infinite cluster incorporates all ionized regions above $\sim8\,h^{-1}\mathrm{cMpc}$. We also quantify the clustering of ionized regions of varying radius with respect to matter and on small scales detect the formation of superbubbles in the overlap phase. On large scales we quantify the bias values of the centres of ionized and neutral regions of different sizes and not only show that the largest ones at the high-point of reionization can reach $b\approx 30$, but also that early small ionized regions are positively correlated with matter and large neutral regions and late small ionized regions are heavily anti-biased with respect to matter, down to $b\lesssim-20$.