Local reionizations histories with merger tree of HII regions
Jonathan Chardin, Dominique Aubert, Pierre Ocvirk

TL;DR
This study investigates the early stages of cosmic reionization around galaxy progenitors using simulations, revealing how HII region properties depend on halo mass and simulation parameters, and predicting reionization histories of present-day galaxies.
Contribution
It introduces a merger tree approach to analyze HII region evolution and provides quantitative predictions on the size and duration of initial HII regions based on halo mass.
Findings
Later HII regions have shorter lifetimes and smaller volumes.
More massive halos have earlier and larger HII regions.
Reionization properties vary with simulation box size and source models.
Abstract
We constrain the initial stage of the reionization process around progenitors of galaxies, such as the extent of the initial HII region before its fusion with the UV background and the duration of its propagation. We use a set of reionisation simulations with different resolutions and ionizing source recipes. A catalog of the HII regions properties is built thanks to a merger tree of HII regions. We draw local reionization histories as a function of time and investigate variations according to the halo mass progenitors of the regions. We then extrapolate the halo mass inside the region from high z to z=0 to make predictions about the reionization histories of z=0 galaxies. We found that the later an HII region appears, the smaller will be its related lifetime and volume before it sees the global UV background. Quantitatively the duration and the extent of the initial growth of an HII…
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