Inside-out or outside-in: the topology of reionization in the photon-starved regime suggested by Lyα forest data

Abstract

We use a set of seminumerical simulations based on the Zel'dovich approximation, the 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 Lyα 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 the shape and amplitude of the power spectrum of fluctuations of 21 cm emission. The slope of the 21 cm power spectrum as measured by upcoming 21 cm 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.

The First Billion Years project: the escape fraction of ionizing photons in the epoch of reionization

“…Bouwens et al 2012;Robertson et al 2013), semi-analytic modelling of reionization (e.g. Choudhury et al 2009;Pritchard et al 2010;Santos et al 2010;Mesinger et al 2011;Raskutti et al 2012;Mitra et al 2013;Shull et al 2012) and numerical simulations of reionization (e.g. Iliev et al 2006; Trac & Cen 2007;Ciardi et al 2012).…”

A study of simulated reionization histories with merger trees of HII regions

“…Barkana & Loeb 2004;Furlanetto et al 2004b;Zahn et al 2007;Mesinger & Furlanetto 2007;Choudhury et al 2009) and simulation techniques (Abel et al 1999;Gnedin & Abel 2001;Razoumov et al 2002;Iliev et al 2006a;Trac & Cen 2007;Aubert & Teyssier 2008;Finlator et al 2009a;Petkova & Springel 2009) have been proposed to investigate the impact of the radiative transfer on the reionization epoch. See Trac & Gnedin (2011) for a complete review of these models.…”

Linear perturbation theory of reionization in position space: Cosmological radiative transfer along the light cone

“…These differences may be due to the fact that both the LPTR and the ESMR models considered here possess a deficiency in how they treat the end of reionization. 7 For example, the "infinite speed of light" assumption implicitly made in the ESMR breaks down near the end of reionization, and the assumption of linearized radiative transfer in LPTR becomes a bad approximation when H II bubbles approach the large scale of interest as they grow. On the other hand, the middle and bottom panels show that their ionized density biases are consistent with each other, aside from the timing difference at the end of reionization.…”

“…In fact, because of this, the LPTR can serve as the basis of a semi-numerical simulation algorithm 8 , which can efficiently generate 3D realizations of H II distributions, in a similar spirit to semi-numerical codes [7][8][9][10] based on the ESMR (Mao et al in prep. ).…”

“…[6] and subsequently developed as the basis of several "semi-numerical" simulation codes [7][8][9][10]. These ESMR-based semi-numerical algorithms have been shown to agree well with full radiative transfer simulations for the simplest models of reionization [11,12].…”