Spokes in ring galaxies

Abstract

Encounters between small companion galaxies and self-gravitating discs containing both stars and gas are examined. In particular, we consider axial collisions like those believed responsible for large-amplitude rings in spiral galaxies. As in earlier studies, the simulations here demonstrate that sufficiently large impulses excite epicyclic motions in discs which yield outwardly propagating waves resembling the structures in ring galaxies. Owing to the effects of dissipation, the gas is subject to fragmentation under its own self-gravity. In some cases, this fragmentation seeds the growth of non-axisymmetric features reminiscent of the `spokes' seen in the Cartwheel ring galaxy. Implications of these findings are discussed along with observational signatures that can be used to test models like those presented here.