  We combine the high-resolution Aquarius simulations with three-dimensional
models of reionization based on the initial density field of the Aquarius
parent simulation, Millennium-II, to study the impact of patchy reionization on
the faint satellite population of Milky Way halos. Because the Aquarius suite
consists of zoom-in simulations of halos in the Millennium-II volume, we follow
the formation of substructure and the growth of reionization bubbles due to the
larger environment simultaneously, and thereby determine the reionization
redshifts of satellite candidates. We do this for four different reionization
models, and also compare results to instantaneous reionization. Using a simple
procedure for selecting satellites and assigning luminosities in the
simulations, we compare the resulting satellite populations. We find that the
overall number of satellites depends sensitively on the reionization model,
with a factor of 3-4 variation between the four models for a given host halo,
although the difference is entirely in the population of faint satellites (M_V
> -10). In addition, we find that for a given reionization model the total
number of satellites differs by 10%-20% between the patchy and homogeneous
scenarios, provided that the redshift is chosen appropriately for the
instantaneous case. However, the halo-halo scatter from the six Aquarius halos
is large, up to a factor of 2-3, and so is comparable to the difference between
reionization scenarios. In order to use the population of faint dwarf galaxies
around the Milky Way as a probe of the local reionization history, then, it is
necessary to first better understand the general distribution of substructure
around Milky Way-mass halos.
