  The reionization of the Universe has profound effects on the way galaxies
form and on their observed properties at later times. Of particular importance
is the relative timing of the reionization history of a region and its halo
assembly history, which can affect the nature of the first stars formed in that
region, the properties and radial distribution of its stellar halo, globular
cluster population and its satellite galaxies. We distinguish two basic cases
for the reionization of a halo - internal reionization, whereby the stars
forming in situ reionize their host galaxy, and external reionization, whereby
the progenitor of a galaxy is reionized by external radiation before its own
stars are able to form in sufficient numbers. We use a set of large-scale
radiative transfer and structure formation simulations, based on cosmologies
derived from both WMAP 1-year and WMAP 3-year data, to evaluate the mean
reionization redshifts and the probability of internal/external reionization
for Local Group-like systems, galaxies in the field and central cD galaxies in
clusters. We find that these probabilities are strongly dependent on the
underlying cosmology and the efficiency of photon production, but also on the
halo mass. There is a rapid transition between predominantly external and
predominantly internal reionization at a mass scale of 1.0e12 Msun
(corresponding roughly to L*galaxies), with haloes less massive than this being
reionized preferentially from distant sources. We provide a fit for the
reionization redshift as a function of halo mass, which could be helpful to
parameterize reionization in semi-analytical models of galaxy formation on
cosmological scales. We find no statistical correlation between the
reionization history of field galaxies and their environment.
