Satellite Dwarf Galaxies in a Hierarchical Universe: The Prevalence of
  Dwarf-Dwarf Major Mergers
  Mergers are a common phenomenon in hierarchical structure formation,
especially for massive galaxies and clusters, but their importance for dwarf
galaxies in the Local Group remains poorly understood. We investigate the
frequency of major mergers between dwarf galaxies in the Local Group using the
ELVIS suite of cosmological zoom-in dissipationless simulations of Milky Way-
and M31-like host halos. We find that ~10% of satellite dwarf galaxies with
M_star > 10^6 M_sun that are within the host virial radius experienced a major
merger of stellar mass ratio closer than 0.1 since z = 1, with a lower fraction
for lower mass dwarf galaxies. Recent merger remnants are biased towards larger
radial distance and more recent virial infall times, because most recent
mergers occurred shortly before crossing within the virial radius of the host
halo. Satellite-satellite mergers also occur within the host halo after virial
infall, catalyzed by the large fraction of dwarf galaxies that fell in as part
of a group. The merger fraction doubles for dwarf galaxies outside of the host
virial radius, so the most distant dwarf galaxies in the Local Group are the
most likely to have experienced a recent major merger. We discuss the
implications of these results on observable dwarf merger remnants, their star
formation histories, the gas content of mergers, and massive black holes in
dwarf galaxies.
