Living with Neighbors. III. Scrutinizing the Spin$-$Orbit Alignment of
  Interacting Dark Matter Halo Pairs
  We present that the spin$-$orbit alignment (SOA; i.e., the angular alignment
between the spin vector of a halo and the orbital angular momentum vector of
its neighbor) provides an important clue to how galactic angular momenta
develop. In particular, we identify virial-radius-wise contact halo pairs with
mass ratios from 1/3 to 3 in a set of cosmological $N$-body simulations, and
divide them into merger and flyby subsamples according to their total
(kinetic+potential) energy. In the spin$-$orbit angle distribution, we find a
significant SOA in that $75.0\pm0.6$ % of merging neighbors and $58.7\pm0.6$ %
of flybying neighbors are on the prograde orbit. The overall SOA of our sample
is mainly driven by fast-rotating halos, corroborating that a well-aligned
interaction spins a halo faster. More interestingly, we find for the first time
a strong number excess of nearly perpendicular but still prograde interactions
($\sim75^{\circ}$) in the spin$-$orbit angle distribution for both the merger
and flyby cases. Such prograde-polar interactions predominate for slow-rotating
halos, testifying that misaligned interactions reduce the halos' spin. The
frequency of the prograde-polar interactions correlates with the halo mass, yet
anticorrelates with the large-scale density. This instantly invokes the
spin-flip phenomenon that is conditional on the mass and environment. The
prograde-polar interaction will soon flip the spin of a slow-rotator to align
with its neighbor's orbital angular momentum. Finally, we propose a scenario
that connects the SOA to the ambient large-scale structure based on the
spin-flip argument.
