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

4. FORMATION PROCESSES

The most general conclusion that can be drawn from a comparison of the dynamical models with the data is that elliptical galaxies show a much narrower range in structures and kinematics than allowed by the models. The clearest examples are the similar light profiles and M / L values, the Faber-Jackson relation, the regular isophote shapes of galaxies, the correlations of v / with luminosity and other parameters, the relatively small kinematic misalignments, the color-magnitude relation (193), etc. These properties are most likely a result of the formation process, and not the equilibrium dynamics. It should be noted in this context that the role of instabilities has not been explored fully.

Our understanding of galaxy formation has not progressed sufficiently to have resulted in a finite number of competing theories and predictions, which can be compared in detail to the observations. Some elements of theories and observations are usually pieced together, often without the basis of a comprehensive model of galaxy formation. Given the lack of predictive power for physical processes involving gas dynamics and star formation, many of the statements we can make are only qualitative.

A fundamental problem with any interpretation is the question of whether ellipticals have dark halos, and if not, if that lack is due to stripping. If ellipticals were born without dark halos, then they are very different from spiral galaxies, and the similarity between bulges and ellipticals would only be superficial. Without any good evidence for such a fundamental distinction, it is probably best to assume that both types of galaxies have dark halos.