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 of 1 Gpc h ^-1 across and a resolution of about 1 Mpc. We achieve this by applying three-dimensional simulations of reionization, based on the extended Press-Schechter formalism, to the same initial conditions as the N-body simulations. We resolve about 2 × 10⁶ halos of mass greater than ~10¹² M _sun at z = 0, and 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 that 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 halo mass is taken as a prior. On average, most halos with mass less than 10¹³ M _sun were reionized internally, while almost all halos with masses greater than 10¹⁴ M _sun 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.