Pre-processing, group accretion and the orbital trajectories of
  associated subhaloes
  We use a high-resolution cosmological dark matter-only simulation to study
the orbital trajectories of haloes and subhaloes in the environs of isolated
hosts. We carefully tally all apsis points and use them to distinguish haloes
that are infalling for the first time from those that occupy more evolved
orbits. We find that roughly 21 per cent of subhaloes within a host's virial
radius are currently on first infall, and have not yet reached their first
orbital pericentre; roughly 44 per cent are still approaching their first
apocentre after infall. For the range of host masses studied, roughly half of
all accreted systems were pre-processed prior to infall, and about 20 per cent
were accreted in groups. We confirm that the entire population of accreted
subhaloes -- often referred to as "associated" subhaloes -- extend far beyond
the virial radii of their hosts, with roughly half currently residing at
distances that exceed $\approx 1.2\times r_{200}$. Many of these backsplash
haloes have gained orbital energy since infall, and occupy extreme orbits that
carry them well past their initial turnaround radii. Such extreme orbits are
created during the initial accretion and dissolution of loosely bound groups,
but also through penetrating encounters between subhaloes on subsequent orbits.
The same processes may also give rise to unexpectedly abrupt losses of orbital
energy. These effects combine, giving rise to a large variation in the ratio of
sequent apocentres for accreted systems. We find that, within 2 virial radii
from host centres, the concentrations of first-infall halos are remarkably
similar those of isolated field halos, whereas backsplash haloes, as well as
systems that were pre-processed, are considerably more concentrated.
