IslandFAST: A Semi-numerical Tool for Simulating the Late Epoch of Reionization

TL;DR

IslandFAST is a semi-numerical simulation tool designed to model the late stages of cosmic reionization, capturing the evolution of neutral islands and the impact of different absorption systems on reionization dynamics.

Contribution

This work introduces islandFAST, a novel semi-numerical simulation specifically tailored for the late epoch of reionization, incorporating the effects of large-scale neutral islands and small-scale absorbers.

Findings

Neutral islands dominate the morphology of the ionization field.

Small scale absorbers significantly reduce the ionizing background.

Reionization proceeds rapidly during the late epoch.

Abstract

We present the algorithm and main results of our semi-numerical simulation, islandFAST, which is developed from the 21cmFAST and designed for the late stage of reionization. The islandFAST predicts the evolution and size distribution of the large scale under-dense neutral regions (neutral islands), and we find that the late Epoch of Reionization (EoR) proceeds very fast, showing a characteristic scale of the neutral islands at each redshift. Using islandFAST, we compare the impact of two types of absorption systems, i.e. the large scale under-dense neutral islands versus small scale over-dense absorbers, in regulating the reionization process. The neutral islands dominate the morphology of the ionization field, while the small scale absorbers dominate the mean free path of ionizing photons, and also delay and prolong the reionization process. With our semi-numerical simulation, the evolution of the ionizing background can be derived self-consistently given a model for the small absorbers. The hydrogen ionization rate of the ionizing background is reduced by an order of magnitude in the presence of dense absorbers.