A study of simulated histories of reionization with merger trees of HII regions

TL;DR

This paper introduces a merger tree methodology to analyze the reionization process in simulations, tracking the evolution of HII regions and comparing different ionizing source models to understand their impact on reionization history.

Contribution

The study presents a novel merger tree approach to analyze reionization, applying it to various source models and resolutions to quantify their effects on HII region growth and reionization timelines.

Findings

Semi-analytical models show homogeneous reionization with hierarchical growth.

Star formation models exhibit fewer, larger dominant HII regions early on.

Increased resolution reduces differences between models.

Abstract

We describe a new methodology to analyze the reionization process in numerical simulations: the chronology and the geometry of reionization is investigated by means of merger histories of individual HII regions. From the merger tree of ionized patches, one can track the individual evolution of the regions properties such as e.g. their size, or the intensity of the percolation process by looking at the formation rate, the frequency of mergers and the number of individual HII regions involved in the mergers. We apply the merger tree technique to simulations of reionization with three different kinds of ionizing source models and two resolutions. Two of them use star particles as ionizing sources. In this case we confront two emissivity evolutions for the sources in order to reach the reionization at z ~ 6. As an alternative we built a semi-analytical model where the dark matter halos extracted from the density fields are assumed as ionizing sources. We then show how this methodology is a good candidate to quantify the impact of the adopted star formation on the history of the observed reionization. The semi-analytical model shows a homogeneous reionization history with 'local' hierarchical growth steps for individual HII regions. On the other hand auto-consistent models for star formation tend to present fewer regions with a dominant region in size which governs the fusion process early in the reionization at the expense of the 'local' reionizations. The differences are attenuated when the resolution of the simulation is increased.