Connecting Reionization to the Local Universe

TL;DR

This study combines N-body simulations and 3D reionization calculations to explore how reionization history correlates with local environment and halo mass, revealing that massive halos reionized earlier and locally, with implications for galaxy formation.

Contribution

It introduces a novel combined simulation approach to link reionization timing with local halo environments and mass, providing new insights into galaxy and cluster formation.

Findings

Cluster halos reionized early and locally within a few Mpc.

Milky Way mass halos reionized later, similar to the global average.

Most halos under 10^13 Msun reionized internally, larger halos by their progenitors.

Abstract

We present results of combined N-body and three-dimensional reionization calculations to determine the relationship between reionization history and local environment in a volume 1 Gpc/h across and a resolution of about 1 Mpc. We resolve the formation of about 2x10^6 halos of mass greater than ~10^12 Msun at z=0, allowing us to determine the relationship between halo mass and reionization epoch for galaxies and clusters. For our fiducial reionization model, in which reionization begins at z~15 and ends by z~6, we find a strong bias for cluster-size halos to be in the regions which reionized first, at redshifts 10<z<15. Consequently, material in clusters was reionized within relatively small regions, on the order of a few Mpc, implying that all clusters in our calculation were reionized by their own progenitors. Milky Way mass halos were on average reionized later and by larger regions, with a distribution similar to the global one, indicating that low mass halos are relatively uncorrelated with reionization when only their mass is taken as a prior. On average, we find that most halos with mass less than 10^13 Msun were reionized internally, while almost all halos with masses greater than 10^14 Msun were reionized by their own progenitors. We briefly discuss the implications of this work in light of the "missing satellites" problem and how this new approach may be extended further.