Minor mergers and their impact on the kinematics of old and young
  stellar populations in disk galaxies
  By means of N-body simulations we investigate the impact of minor mergers on
the angular momentum and dynamical properties of the merger remnant. Our
simulations cover a range of initial orbital characteristics and gas-to-stellar
mass fractions (from 0 to 20%), and include star formation and supernova
feedback. We confirm and extend previous results by showing that the specific
angular momentum of the stellar component always decreases independently of the
orbital parameters or morphology of the satellite, and that the decrease in the
rotation velocity of the primary galaxy is accompanied by a change in the
anisotropy of the orbits. However, the decrease affects only the old stellar
population, and not the new population formed from gas during the merging
process. This means that the merging process induces an increasing difference
in the rotational support of the old and young stellar components, with the old
one lagging with respect to the new. Even if our models are not intended
specifically to reproduce the Milky Way and its accretion history, we find
that, under certain conditions, the modeled rotational lag found is compatible
with that observed in the Milky Way disk, thus indicating that minor mergers
can be a viable way to produce it. The lag can increase with the vertical
distance from the disk midplane, but only if the satellite is accreted along a
direct orbit, and in all cases the main contribution to the lag comes from
stars originally in the primary disk rather than from stars in the satellite
galaxy. We also discuss the possibility of creating counter-rotating stars in
the remnant disk, their fraction as a function of the vertical distance from
the galaxy midplane, and the cumulative effect of multiple mergers on their
creation.
