Gas Dynamics in a Tidal Interaction: Formation of Rings

Abstract

Some ring galaxies, of which the Cartwheel is the archetype, form during the near-central head-on intrusion of a small companion in a disk galaxy (Lynds and Toomre, 1976; Theys and Spiegel 1976). Such rings are of essentially stellar nature. On the contrary, many spiral galaxies exhibit ring-like features that mostly consist of gas and young stars and do not involve the old stellar population. Among the latter, we shall only consider objects with nuclear rings, located at the turnover of the rotation curve, usually within the inner kiloparsec. In particular, we shall leave apart the galaxies which have annular distributions of gas, at radii around 4–5 kpc like our Galaxy, M31, and M81, since they may have a different origin.