Andromeda II as a merger remnant
  Using N-body simulations we study the origin of prolate rotation recently
detected in the kinematic data for And II, a dSph satellite of M31. We propose
an evolutionary model for the origin of And II involving a merger between two
disky dwarf galaxies whose structural parameters differ only in their disk
scale lengths. The dwarfs are placed on a radial orbit towards each other with
their angular momenta inclined by 45 deg to the orbital plane and by 90 deg
with respect to each other. After 5 Gyr of evolution the merger remnant forms a
stable triaxial galaxy with rotation only around the longest axis. The origin
of this rotation is naturally explained as due to the symmetry of the initial
configuration which leads to the conservation of angular momentum components
along the direction of the merger. The stars originating from the two dwarfs
show significantly different surface density profiles while having very similar
kinematics in agreement with the properties of separate stellar populations in
And II. We also study an alternative scenario for the formation of And II, via
tidal stirring of a disky dwarf galaxy. While intrinsic rotation occurs
naturally in this model as a remnant of the initial rotation of the disk, it is
mostly around the shortest axis of the stellar component. The rotation around
the longest axis is induced only occasionally and remains much smaller that the
system's velocity dispersion. We conclude that although tidal origin of the
velocity distribution in And II cannot be excluded, it is much more naturally
explained within the scenario involving a past merger event. Thus, in
principle, the presence of prolate rotation in dSph galaxies of the Local Group
and beyond may be used as an indicator of major mergers in their history or
even as a way to distinguish between the two scenarios of their formation.
