The multiplicity and anisotropy of galactic satellite accretion
  We study the incidence of group and filamentary dwarf galaxy accretion into
Milky Way (MW) mass haloes using two types of hydrodynamical simulations:
EAGLE, which resolves a large cosmological volume, and the AURIGA suite, which
are very high resolution zoom-in simulations of individual MW-sized haloes. The
present-day 11 most massive satellites are predominantly (75%) accreted in
single events, 14% in pairs and 6% in triplets, with higher group
multiplicities being unlikely. Group accretion becomes more common for fainter
satellites, with 60% of the top 50 satellites accreted singly, 12% in pairs,
and 28% in richer groups. A group similar in stellar mass to the Large
Magellanic Cloud (LMC) would bring on average 15 members with stellar mass
larger than $10^4{~\rm M_\odot}$. Half of the top 11 satellites are accreted
along the two richest filaments. The accretion of dwarf galaxies is highly
anisotropic, taking place preferentially perpendicular to the halo minor axis,
and, within this plane, preferentially along the halo major axis. The satellite
entry points tend to be aligned with the present-day central galaxy disc and
satellite plane, but to a lesser extent than with the halo shape. Dwarfs
accreted in groups or along the richest filament have entry points that show an
even larger degree of alignment with the host halo than the full satellite
population. We also find that having most satellites accreted as a single group
or along a single filament is unlikely to explain the MW disc of satellites.
