Annu. Rev. Astron. Astrophys. 1991. 29: 239-274 Copyright © 1991 by Annual Reviews. All rights reserved

2.7 Dissipationless Collapse

Models for elliptical galaxies have been constructed also by means of N-body simulations, starting from initial conditions out of virial equilibrium (i.e., initial kinetic energy T smaller than half the potential energy W). The resulting collapse generally produces a triaxial final state. Early work concentrated on establishing the existence of these systems (2, 248, 250, 251). Subsequent studies have addressed their internal dynamical structure (341, 371), including the growth of a central black hole (263), and the formation of shells (20, 281, 282).

In a seminal paper, van Albada (340) showed that large collapse factors (2T / W 0.1) and clumpy initial conditions are needed to produce (spherical) models with realistic r^1/4 density profiles (cf. 232, 358), and that the properties of the final state depend on the dynamical constraints imposed during the collapse (229), as expected for incomplete violent relaxation (222, 324). Follow-up work has concentrated on better understanding the physics of this process, by using simple considerations to identify the probable form of the resulting distribution function (39, 174, 247, 323, 335), or by invoking entropy arguments (72, 336, 364, 367). In simulations with larger collapse factors (2T / W < 0.1), the radial orbit instability is important (243). The resulting final states are strongly prolate/triaxial (3, 4, 216, 217, 269). They resemble elliptical galaxies in some respects, but their flattenings are somewhat larger than observed (Section 3.4). The presence of a dark halo does not change this result significantly (325).