Ecology of dark matter haloes -II. Effects of interactions on the
  alignment of halo pairs
  We use the Horizon Run 4 cosmological N -body simulation to study the effects
of distant and close interactions on the alignments of the shapes, spins, and
orbits of targets haloes with their neighbours, and their dependence on the
local density environment and neighbour separation. Interacting targets have a
significantly lower spin and higher sphericity and oblateness than all targets.
Interacting pairs initially have anti-parallel spins, but the spins develop
parallel alignment as time goes on. Neighbours tend to evolve in the plane of
rotation of the target, and in the direction of the major axis of prolate
haloes. Moreover, interactions are preferentially radial, while pairs with
non-radial orbits are preferentially prograde. The alignment signals are
stronger at high-mass and for close separations, and independent on the
large-scale density. Positive alignment signals are found at redshifts up to 4,
and increase with decreasing redshifts. Moreover, the orbits tend to become
prograde at low redshift, while no alignment is found at high redshift (z = 4).
