The Lyman-limit photon mean free path at the end of late reionization in the Sherwood–Relics simulations

Abstract

ABSTRACT Recent evidence supporting reionization ending at redshift $z \lesssim 6$ includes the rapid redshift evolution of the mean free path, $\lambda _{\rm mfp}$, for Lyman-limit photons through the intergalactic medium (IGM) at $5\lt z\lt 6$. Here, we investigate $\lambda _{\rm mfp}$ predicted by the Sherwood–Relics suite of hybrid radiation hydrodynamical simulations. Simulations with comoving volumes of $40^{3}\, h^{-3}\rm \, cMpc^{3}$ ($160^{3}\, h^{-3}\rm \, cMpc^{3}$), calibrated to match the observed Ly $\alpha$ forest transmission with a late end to reionization at $z\lt 6$, are consistent with recent $\lambda _{\rm mfp}$ measurements at $z\lt 5.9$, and are $1.2\sigma$ ($1.8\sigma$) above the highest redshift $\lambda _{\rm mfp}$ measurement at $z=5.93$. The majority of the Lyman-limit opacity at the end of reionization is attributable to highly ionized Ly $\alpha$ forest absorbers with neutral hydrogen column densities $N_{\rm HI} \leq 10^{16}$–$10^{17}\rm \, cm^{-2}$. Resolving these systems is critical for capturing the redshift evolution of $\lambda _{\rm mfp}$ during the final stages of reionization. After reionization completes, overdense gas will reduce $\lambda _{\rm mfp}$ by up to 20 per cent around haloes with masses $M_{\rm h}\sim 10^{9}$–$10^{11}h^{-1}\, {\rm M}_{\odot }$, but during reionization ionized bubbles will instead boost $\lambda _{\rm mfp}$ around haloes by up to an order of magnitude when the IGM is as much as 90 per cent neutral by volume. This effect will play an important role in the visibility of Ly $\alpha$ emitting galaxies at $z\gtrsim 10$ discovered with JWST.