The resonant nature of tidal stirring of disky dwarf galaxies orbiting the Milky Way

TL;DR

This study uses N-body simulations to demonstrate that the tidal evolution of disky dwarf galaxies orbiting the Milky Way is strongly dependent on the orientation of their disks, with resonant effects playing a key role.

Contribution

It reveals the significant impact of disk orientation on tidal evolution and highlights the importance of resonant effects in shaping dwarf galaxy morphology.

Findings

Prograde orbits lead to strong tidal effects and bar formation.

Retrograde orbits result in minimal evolution and slow rotation loss.

Intermediate orientations show a monotonic transition in evolution.

Abstract

Using N-body simulations we study the tidal evolution of initially disky dwarf galaxies orbiting a Milky Way-like host, a process known to lead to the formation of dwarf spheroidal galaxies. We focus on the effect of the orientation of the dwarf galaxy disk's angular momentum with respect to the orbital one and find very strong dependence of the evolution on this parameter. We consider four different orientations: the exactly prograde, the exactly retrograde and two intermediate ones. Tidal evolution is strongest for the exactly prograde and weakest for the exactly retrograde orbit. In the prograde case the stellar component forms a strong bar and remains prolate until the end of the simulation, while its rotation is very quickly replaced by random motions of the stars. In the retrograde case the dwarf remains oblate, does not form a bar and loses rotation very slowly. In the two cases of intermediate orientation of the disk, the evolution is between the two extremes, suggesting a monotonic dependence on the inclination. We interpret the results in terms of the resonance between the angular velocity of the stars in the dwarf and its orbital motion by comparing the measurements from simulations to semi-analytic predictions. We conclude that resonant effects are the most important mechanism underlying the tidal evolution of disky dwarf galaxies.