Stability of Elliptical Galaxies. Numerical Experiments

Abstract

The idea that dynamical instabilities might play an important role in determining the equilibrium structure of elliptical galaxies is a startling one, especially to those of us who are accustomed to associating instabilities with rapidly-rotating systems like disk galaxies. The shock is even greater when we learn that these instabilities have been taken seriously by workers in the Soviet Union for a long time. As Dr. Polyachenko points out in his talk, instabilities affecting spherical, non- rotating galaxies were being discussed by Soviet astronomers as long ago as 1972. Much of this work has recently become more accessible through the publication of an English-language version of Fridman and Polyachenko’s monograph, Physics of Gravitating Systems (Fridman and Polyachenko 1984). In the West, it appears that only two people were prescient enough to systematically test the stability of spherical models before learning of the Soviet work (Hénon 1973; Barnes 1985). In particular, Barnes discovered independently that a spherical system composed of predominantly radial orbits evolves rapidly into a bar. Subsequent work has demonstrated that even some mildly anisotropic models can be unstable in this way.