# Neutral island statistics during reionization from 21-cm tomography

## TL;DR

This paper investigates the properties of neutral regions during reionization using 21-cm tomography, showing how these features can distinguish different reionization scenarios and relate to observable signals.

## Contribution

It introduces a method to analyze neutral islands in 21-cm data, highlighting their potential to differentiate reionization models and connect to observational features.

## Key findings

- Neutral islands are fewer but larger than ionized bubbles at similar volume fractions.
- The size distribution and percolation behavior of islands differ significantly from ionized regions.
- Long neutral regions (>100 Mpc) persist until late reionization stages, relevant for quasar absorption spectra.

## Abstract

We present the prospects of extracting information about the Epoch of Reionization by identifying the remaining neutral regions, referred to as islands, in tomographic observations of the redshifted 21-cm signal. Using simulated data sets we show that at late times the 21-cm power spectrum is fairly insensitive to the details of the reionization process but that the properties of the neutral islands can distinguish between different reionization scenarios. We compare the properties of these islands with those of ionized bubbles. At equivalent volume filling fractions, neutral islands tend to be fewer in number but larger compared to the ionized bubbles. In addition, the evolution of the size distribution of neutral islands is found to be slower than that of the ionized bubbles and also their percolation behaviour differs substantially. Even though the neutral islands are relatively rare, they will be easier to identify in observations with the low-frequency component of the Square Kilometre Array (SKA-Low) due to their larger size and the lower noise levels at lower redshifts. The size distribution of neutral islands at the late stages of reionization is found to depend on the source properties, such as the ionizing efficiency of the sources and their minimum mass. We find the longest line of sight through a neutral region to be more than 100 comoving Mpc until very late stages (90-95 per cent reionized), which may have relevance for the long absorption trough at $z=5.6-5.8$ in the spectrum of quasar ULAS J0148+0600.