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Bars in disky galaxies can be formed by interactions with other systems,including those of comparable mass. It has long been established that theeffect of such interactions on galaxy morphology depends strongly on theorbital configuration, in particular the orientation of the intrinsic spin ofthe galactic disk with respect to its orbital angular momentum. Progradeencounters modify the morphology strongly, including the formation of tidallyinduced bars, while retrograde flybys should have little effect on morphology.Recent works on the subject reached conflicting conclusions, one using theimpulse approximation and claiming no dependence on this angle in theproperties of tidal bars. To resolve the controversy, we performedself-consistent N-body simulations of hyperbolic encounters between twoidentical Milky Way-like galaxies assuming different velocities and impactparameters, with one of the galaxies on a prograde and the other on aretrograde orbit. The galaxies were initially composed of an exponentialstellar disk and an NFW dark halo, and were stable against bar formation inisolation for 3 Gyr. We find that strong tidally induced bars form only ingalaxies on prograde orbits. For smaller impact parameters and lower relativevelocities the bars are stronger and have lower pattern speeds. Stronger barsundergo extended periods of buckling instability that thicken their verticalstructure. The encounters also lead to the formation of two-armed spirals withstrength inversely proportional to the strength of the bars. We conclude thatproper modeling of prograde and retrograde encounters cannot rely on thesimplest impulse approximation.

Formation of tidally induced bars in galactic flybys: prograde versus retrograde encounters