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

TL;DR

This study uses self-consistent N-body simulations to demonstrate that tidally induced bars in galaxies form only during prograde encounters, with retrograde flybys having little to no effect on bar formation.

Contribution

It provides a detailed simulation-based analysis showing the dependence of tidally induced bar formation on orbital orientation, clarifying previous conflicting results.

Findings

Strong bars form only in prograde encounters.

Bar strength correlates with impact parameter and velocity.

Bars often undergo buckling instability, thickening vertically.

Abstract

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.