Evolution of the spin of late-type galaxies caused by galaxy-galaxy
  interactions
  We use N-body/hydrodynamic simulations to study the evolution of the spin of
a Milky Way-like galaxy through interactions. We perform a controlled
experiment of co-planner galaxy-galaxy encounters and study the evolution of
disk spins of interacting galaxies. Specifically, we consider the cases where
the late-type target galaxy encounters an equally massive companion galaxy,
which has either a late or an early-type morphology, with the closest approach
distance of about 50 kpc, in prograde or retrograde sense. By examining the
time change of the circular velocity of the disk material of the target galaxy
from each case, we find that the target galaxy tends to lose the spin through
prograde collisions but hardly through retrograde collisions, regardless of the
companion galaxy type. The decrease of the spin results mainly from the
deflection of the orbit of the disk material by tidal disruption. Although
there is some disk material which gains the circular velocity through
hydrodynamic as well as gravitational interactions or by transferring material
from the companion galaxy, it turns out that the amount of the material is
generally insufficient to increase the overall galactic spin under the
conditions we set. It is found that the spin angular momentum of the target
galaxy disk decreases by 15 - 20% after a prograde collision. We conclude that
the accumulated effects of galaxy-galaxy interactions will play an important
role in determining the total angular momentum of late-type galaxies at current
stage.
