Effects of Small-Scale Absorption Systems on Neutral Islands during the Late Epoch of Reionization

TL;DR

This study uses semi-numerical simulations to explore how small-scale absorbers influence the size and evolution of neutral islands during the late Epoch of Reionization, with implications for upcoming 21-cm observations.

Contribution

It introduces a comparative analysis of different SSA models and their impact on neutral island evolution during reionization, highlighting a new observational approach.

Findings

Neutral island size remains stable in no-SSA and moderate-SSA models during late reionization.

In dense-SSA models, large islands break into smaller ones, showing evolution.

Different SSA models can be distinguished via 21-cm imaging and power spectrum measurements.

Abstract

The reionization process is expected to be prolonged by the small-scale absorbers (SSAs) of ionizing photons, which have been seen as Lyman-limit systems in quasar absorption line observations. We use a set of semi-numerical simulations to investigate the effects of absorption systems on the reionization process, especially their impacts on the neutral islands during the late Epoch of Reionization (EoR). Three models are studied, i.e. the extreme case of no-SSA model with a high level of ionizing background, the moderate-SSA model with a relatively high level of ionizing background, and the dense-SSA model with a low level of ionizing background. We find that while the characteristic scale of neutral regions decreases during the early and middle stages of reionization, it stays nearly unchanged at about 10 comoving Mpc during the late stage for the no-SSA and moderate-SSA models. However, in the case of weak ionizing background in the dense-SSA model, the characteristic island scale shows obvious evolution, as large islands break into many small ones that are slowly ionized. The evolutionary behavior of neutral islands during the late EoR thus provides a novel way to constrain the abundance of SSAs. We discuss the 21-cm observation with the upcoming Square Kilometre Array. The different models can be distinguished by either the 21-cm imaging or the 21-cm power spectrum measurements.