The slowing down of galaxy disks in dissipationless minor mergers
  We have investigated the impact of dissipationless minor galaxy mergers on
the angular momentum of the remnant. Our simulations cover a range of initial
orbital characteristics and the system consists of a massive galaxy with a
bulge and disk merging with a much less massive (one-tenth or one-twentieth)
gasless companion which has a variety of morphologies (disk- or
elliptical-like) and central baryonic mass concentrations. During the process
of merging, the orbital angular momentum is redistributed into the internal
angular momentum of the final system; the internal angular momentum of the
primary galaxy can increase or decrease depending on the relative orientation
of the orbital spin vectors (direct or retrograde), while the initially
non-rotating dark matter halo always gains angular momentum. The specific
angular momentum of the stellar component always decreases independent of the
orbital parameters or morphology of the satellite, the decrease in the rotation
velocity of the primary galaxy is accompanied by a change in the anisotropy of
the orbits, and the ratio of rotation speed to velocity dispersion of the
merger remnant is lower than the initial value, not only due to an increase in
the dispersion but also to the slowing -down of the disk rotation. We briefly
discuss several astrophysical implications of these results, suggesting that
minor mergers do not cause a "random walk" process of the angular momentum of
the stellar disk component of galaxies, but rather a steady decrease. Minor
mergers may play a role in producing the large scatter observed in the
Tully-Fisher relation for S0 galaxies, as well as in the increase of the
velocity dispersion and the decrease in $v/\sigma$ at large radii as observed
in S0 galaxies.
