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 × 10 6 halos of mass greater than ∼ 10 12 M ⊙ 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 10 13 M ⊙ were reionized internally, while almost all halos with masses greater than 10 14 M ⊙ 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.