Formation of tidally induced bars in galactic flybys: prograde versus
  retrograde encounters
  Bars in disky galaxies can be formed by interactions with other systems,
including those of comparable mass. It has long been established that the
effect of such interactions on galaxy morphology depends strongly on the
orbital configuration, in particular the orientation of the intrinsic spin of
the galactic disk with respect to its orbital angular momentum. Prograde
encounters modify the morphology strongly, including the formation of tidally
induced bars, while retrograde flybys should have little effect on morphology.
Recent works on the subject reached conflicting conclusions, one using the
impulse approximation and claiming no dependence on this angle in the
properties of tidal bars. To resolve the controversy, we performed
self-consistent N-body simulations of hyperbolic encounters between two
identical Milky Way-like galaxies assuming different velocities and impact
parameters, with one of the galaxies on a prograde and the other on a
retrograde orbit. The galaxies were initially composed of an exponential
stellar disk and an NFW dark halo, and were stable against bar formation in
isolation for 3 Gyr. We find that strong tidally induced bars form only in
galaxies on prograde orbits. For smaller impact parameters and lower relative
velocities the bars are stronger and have lower pattern speeds. Stronger bars
undergo extended periods of buckling instability that thicken their vertical
structure. The encounters also lead to the formation of two-armed spirals with
strength inversely proportional to the strength of the bars. We conclude that
proper modeling of prograde and retrograde encounters cannot rely on the
simplest impulse approximation.
