Connecting Reionization to the Local Universe

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−1 across and a resolution of about 1 Mpc. We resolve the formation of about 2× 106 halos of mass greater than ∼ 10M⊙ 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 most similar to the global one, indicating that low mass halos are nearly uncorrelated with reionization when only their mass is taken as a prior. On average, we find that most halos with mass less than 10M⊙ were reionized internally, while almost all halos with masses greater than 10M⊙ 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. Subject headings: cosmology: theory — galaxies: formation — galaxies: intergalactic medium