Tidal evolution of disky dwarf galaxies: prograde versus retrograde orbits

TL;DR

This study uses N-body simulations to explore how the orientation of a dwarf galaxy’s disk affects its tidal transformation into a spheroid when orbiting a larger galaxy, revealing orientation-dependent evolutionary paths.

Contribution

It demonstrates that disk orientation critically influences tidal evolution, with prograde orbits forming bars and rotating, while retrograde orbits remain oblate and slowly lose rotation.

Findings

Prograde orbits produce strong bars and prolate shapes.

Retrograde orbits retain oblate shapes and slow rotation loss.

Resonant effects dominate the evolution process.

Abstract

The formation of dwarf spheroidal galaxies in the Local Group from disky progenitors via tidal interaction with a bigger host is one of the most promising scenarios of their origin. Using N-body simulations we study the process by following the evolution of a disky dwarf orbiting a Milky Way-like host. We focus on the effect of the orientation of the dwarf galaxy disk's angular momentum with respect to the orbital one. We find a strong dependence of the efficiency of the transformation from a disk to a spheroid on the disk orientation. The effect 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 evolution, 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. Our results suggest that resonant effects are the most important mechanism underlying the evolution while tidal shocking plays only a minor role.