|Annu. Rev. Astron. Astrophys. 1991. 29:
Copyright © 1991 by . All rights reserved
There are many outstanding questions regarding the structure, dynamics and formation of elliptical galaxies. What are their intrinsic shapes? Do they have massive halos? Do they contain supermassive central black holes? What is the origin of the r1/4 law? What is the origin of the ``fundamental plane''? Are ellipticals dynamically similar to the bulges, or the halos, of spiral galaxies? What causes the distinction between ellipticals and spirals? When were elliptical galaxies formed, and on what time-scale? Was the main formation mechanism dissipationless, or did it involve gaseous processes? Was the substructure that is observed imprinted during formation, or is it more recent?
Historically, elliptical galaxies were thought to be dynamically simple, one-component collisionless stellar systems with a simple shape, containing very little gas (e.g., 130, 146). The big breakthrough came in the mid-seventies, when it became clear that rather than being boring dynamical systems, elliptical galaxies exhibit a rich variety of shapes and internal dynamics. They are not oblate isotropic rotators, but slowly tumbling triaxial stellar systems whose internal structure is not determined by just their total mass M, and total angular momentum L (32, 34, 35, 36, 169). Thus, the observable properties of nearby elliptical galaxies contain information about their formation, and in principle hold the key to answering many of the above questions. A discussion of the current state of knowledge in this area is the topic of this review.
The structure of elliptical galaxies was reviewed previously by Gott (146) and Binney (38). More recent work can be found in the proceedings of IAU Symposium 127, which was devoted entirely to elliptical galaxies (83). The properties of ellipticals form too wide a topic to be reviewed in one paper. We refer readers interested in surface photometry, X-ray observations, and mergers to other reviews in this series (193, 111, and 20, respectively). Here we focus on the equilibrium dynamics of elliptical galaxies. In Section 2 we summarize recent work on dynamical models, with emphasis on triaxial systems. Observational efforts are reviewed in Section 3. Implications for the formation processes are discussed in Section 4. Finally, in Section 5 we outline possible future work.