The Topology of Cosmological Reionization

TL;DR

This study uses a large cosmological simulation to analyze the topology of neutral hydrogen during reionization, revealing distinct phases and an inside-out ionization pattern consistent with Gaussian randomness after reionization.

Contribution

It introduces the use of the genus curve to quantify the topology of hydrogen distribution during reionization on large scales, identifying four distinct topological phases.

Findings

Reionization proceeds inside-out, with high-density regions ionized first.

Four topological phases identified: pre-reionization, pre-overlap, overlap, post-overlap.

Post-reionization topology matches Gaussian random phase, aligning with observations.

Abstract

Using the largest cosmological reionization simulation to-date (~24 billion particles), we use the genus curve to quantify the topology of neutral hydrogen distribution on scales > 1 Mpc as it evolves during cosmological reionization. We find that the reionization process proceeds primarily in an inside-out fashion, where higher density regions become ionized earlier than lower density regions. There are four distinct topological phases: (1) Pre-reionization at z ~ 15, when the genus curve is consistent with a Gaussian density distribution. (2) Pre-overlap at 10 < z < 15, during which the number of HII bubbles increases gradually with time, until percolation of HII bubbles starts to take effect, characterized by a very flat genus curve at high volume fractions. (3) Overlap at 8 < z < 10, when large HII bubbles rapidly merge, manifested by a precipitous drop in the amplitude of the genus curve. (4) Post-overlap at 6 < z < 8, when HII bubbles have mostly overlapped and the genus curve is consistent with a diminishing number of isolated neutral islands. After the end of reionization (z < 6), the genus of neutral hydrogen is consistent with Gaussian random phase, in agreement with observations.