Inside-out or Outside-in: The topology of reionization in the photon-starved regime suggested by Lyman-alpha forest data

TL;DR

This study uses semi-numerical simulations to show that considering the inhomogeneous distribution of recombination sinks reveals an outside-in reionization topology in the photon-starved regime, affecting 21cm fluctuation signals.

Contribution

It demonstrates the importance of inhomogeneous recombination sinks in shaping reionization topology and 21cm power spectrum predictions, highlighting differences from traditional models.

Findings

Ignoring recombination inhomogeneity leads to misleading reionization topology conclusions.

Reionization in the photon-starved regime is more outside-in than previously thought.

21cm power spectrum slope can distinguish reionization topology.

Abstract

We use a set of semi-numerical simulations based on Zel'dovich approximation, friends-of-friends algorithm and excursion set formalism to generate reionization maps of high dynamic range with a range of assumptions regarding the distribution and luminosity of ionizing sources and the spatial distribution of sinks for the ionizing radiation. We find that ignoring the inhomogeneous spatial distribution of regions of high gas density where recombinations are important -- as is often done in studies of this kind -- can lead to misleading conclusions regarding the topology of reionization, especially if reionization occurs in the photon-starved regime suggested by Lya forest data. The inhomogeneous spatial distribution of recombinations significantly reduces the mean free path of ionizing photons and the typical size of coherently ionized regions. Reionization proceeds then much more as an outside-in process. Low-density regions far from ionizing sources become ionized before regions of high gas density not hosting sources of ionizing radiation. The spatial distribution of sinks of ionization radiation also significantly affects shape and amplitude the power spectrum of fluctuations of 21cm emission. The slope of the 21cm power spectrum as measured by upcoming 21cm experiments should be able to distinguish to what extent the topology of reionization proceeds outside-in or inside-out while the evolution of the amplitude of the power spectrum with increasing ionized mass fraction should be sensitive to the spatial distribution and the luminosity of ionizing sources.