Stability of Nonrotating Stellar Systems. II. Prolate Shell-Orbit Models
Abstract
We investigate the dynamical stability of nonrotating prolate galaxy models constructed from thin long-axis tube orbits ("shell" orbits). Models more elongated than ~E6 (axis ratio of ~2:5) are unstable to bending modes that rapidly increase the velocity dispersion perpendicular to the long axis and decrease the model's elongation We obtain approximate representations of the spatial forms of the fastest growing modes and their growth rates. Most of the evolution is due to two modes: a symmetric (banana-shaped) bending and an antisymmetric (S-shaped) bending. The instability is similar to the "firehose" instability of a thin self-gravitating slab, except that it persists in models with velocity anisotropies that are much less extreme than the critical value for instability of the slab. We present a simple model that reproduces the basic features of the instability in the prolate geometry. Our results provide support for the hypothesis of Fridman and Polyachenko that the absence of elliptical galaxies flatter than about E6 is due to dynamical instability.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- August 1991
- DOI:
- 10.1086/170293
- Bibcode:
- 1991ApJ...376..439M
- Keywords:
-
- Galactic Rotation;
- Galactic Structure;
- Shell Galaxies;
- Stellar Systems;
- Astronomical Models;
- Galactic Evolution;
- Many Body Problem;
- Systems Stability;
- Astrophysics;
- GALAXIES: INTERNAL MOTIONS;
- GALAXIES: STRUCTURE;
- INSTABILITIES;
- STARS: STELLAR DYNAMICS