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 (2*T / 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 (2*T / 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).