Satellite Dwarf Galaxies in a Hierarchical Universe: Infall Histories,
  Group Preprocessing, and Reionization
  In the Local Group (LG), almost all satellite dwarf galaxies that are within
the virial radius of the Milky Way (MW) and Andromeda (M31) exhibit strong
environmental influence. The orbital histories of these satellites provide the
key to understanding the role of the MW/M31 halo, lower-mass groups, and cosmic
reionization on the evolution of dwarf galaxies. We examine the virial-infall
histories of satellites with M_star = 10^{3-9} M_sun using the ELVIS suite of
cosmological zoom-in dissipationless simulations of 48 MW/M31-like halos.
Satellites at z = 0 fell into the MW/M31 halos typically 5 - 8 Gyr ago at z =
0.5 - 1. However, they first fell into any host halo typically 7 - 10 Gyr ago
at z = 0.7 - 1.5. This difference arises because many satellites experienced
"group preprocessing" in another host halo, typically of M_vir ~ 10^{10-12}
M_sun, before falling into the MW/M31 halos. Satellites with lower mass and/or
those closer to the MW/M31 fell in earlier and are more likely to have
experienced group preprocessing; half of all satellites with M_star < 10^6
M_sun were preprocessed in a group. Infalling groups also drive most
satellite-satellite mergers within the MW/M31 halos. Finally, none of the
surviving satellites at z = 0 were within the virial radius of their MW/M31
halo during reionization (z > 6), and only < 4% were satellites of any other
host halo during reionization. Thus, effects of cosmic reionization versus
host-halo environment on the formation histories of surviving dwarf galaxies in
the LG occurred at distinct epochs, separated typically by 2 - 4 Gyr, so they
are separable theoretically and, in principle, observationally.
