| Annu. Rev. Astron. Astrophys. 1991. 29:
239-274
Copyright © 1991 by Annual Reviews. All rights
reserved
|
2.2 Triaxial Models without Figure Rotation
2.2.1 EXISTENCE
Triaxial potentials generally admit only one
exact isolating integral, the orbital energy E. Although there are three
planes of reflection symmetry, there are no symmetry axes, and no component of
the angular momentum vector is conserved. In a pioneering study, Schwarzschild
(305,
306)
showed by numerical orbit calculations
that in triaxial potentials relevant for elliptical galaxies most stellar
orbits possess two effective integrals, I2 and
I3, in addition to the
energy. The fraction of irregular orbits is small
(145).
As a result, most orbits belong to one of only a few major families: box
orbits, short-axis tube orbits, and long-axis tube orbits
(80,
198).
Tube orbits around the intermediate axis are unstable
(153).
The long-axis tubes come in two varieties,
bringing the total number of major orbit families to four. Schwarzschild
showed for a specific triaxial mass model - first with a stationary figure,
and subsequently with a tumbling figure - that it is possible to combine the
individual orbital densities in the associated gravitational potential so that
they reproduce the original mass model. This is equivalent to finding a
distribution function f that is consistent with the mass model
(348),
and hence Schwarzschild's work demonstrated that
self-consistent triaxial galaxy models exist, with and without figure
rotation. Similar conclusions were reached on the basis of N-body simulations
(2,
248,
250,
251,
371,
Section 2.7).
2.2.2 NON-UNIQUENESS
The motion of a star on a box orbit in a
non-rotating triaxial potential is a combination of oscillations in the three
principal directions, so the orbit-averaged angular momentum vanishes. Stars
on tube orbits have a definite sense of rotation around either the long axis
or the short axis of the model. Clockwise and counterclockwise motion may
occur in the same tube orbit. Because the fraction of direct versus retrograde
stars may be chosen freely, the total angular momentum vector of a stationary
triaxial model may be misaligned from the principal axes of the system:
it may point anywhere in the plane containing both the long and the short
axis. We shall see in Section 3.4 that
there is evidence for such a misalignment
in the kinematics of elliptical galaxies.
It is also possible that different combinations of orbits with truly distinct
shapes produce the same triaxial density distribution. Thus, there is a large
degree of non-uniqueness in the distribution functions consistent with a given
three-dimensional mass model. The purpose of the recent work on triaxial
models is to explore this freedom in model building and to construct large
sets of models which can be compared to observations. The main questions are:
what are the permitted intrinsic shapes, figure rotation rates, and streaming
velocities, and what constraints on the structure of elliptical galaxies can
be deduced from detailed observations? We are still far from answering these
questions satisfactorily, but many of the necessary tools have been developed.
Specifically, much can be learnt from a study of special models for which
sufficient simplification occurs so that whole families of them can be
studied. Two useful classes of such models are known. These are the separable
or Stäckel models, and the scale-free models. We discuss each of these in
turn.