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|Title:||Stability of nonrotating stellar systems. II - Prolate shell-orbit models|
|Authors:||Merritt, David; Hernquist, Lars|
|Affiliation:||AA(Rutgers University, Piscataway, NJ), AB(Institute for Advanced Study, Princeton, NJ)|
|Publication:||Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 376, Aug. 1, 1991, p. 439-457. Research supported by University of California and Pittsburgh Supercomputing Center. (ApJ Homepage)|
|NASA/STI Keywords:||Galactic Rotation, Galactic Structure, Shell Galaxies, Stellar Systems, Astronomical Models, Galactic Evolution, Many Body Problem, Systems Stability|
The dynamical stability of nonrotating prolate galaxy models constructed
from thin long-axis tube orbits ('shell' orbits) are investigated.
Models more elongated than about E6 (axis ratio of about 2:5) are
unstable to bending modes than rapidly increase the velocity dispersion
perpendicular to the long axis and decrease the model's elongation.
Approximate representations of the spatial forms of the fastest growing
modes and their growth rates are obtained. 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. A simple model is given that
reproduces the basic features of the instability in the prolate
geometry. These results provide support for the hypothesis of Fridman
and Polyachenko (1984) that the absence of elliptical galaxies flatter
than about E6 is due to dynamical instability.
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