|Annu. Rev. Astron. Astrophys. 1982. 20:
Copyright © 1982 by Annual Reviews. All rights reserved
Elliptical galaxies are the simplest type of galaxy and therefore the natural starting point for studies of galaxies in general. Furthermore, it has often been argued (e.g. de Vaucouleurs 1959, Ostriker 1977) that at the heart of every disk galaxy there sits a small elliptical, the bulge, around which the disk that now dominates the light distribution has been assembled. So it is possible that ellipticals and bulges are, so to speak, the founding fathers of the realm of the nebulae, and as such may have profoundly influenced the form of the components into which the majority of the luminous matter in the Universe subsequently settled.
For many years neither elliptical galaxies nor bulges received the attention warranted by their intrinsic interest. There were two reasons for this. The subsidiary reason was that it was widely felt that elliptical galaxies are so simple that their structures could be inferred by Plato's preferred method of research - pure thought. Surely these systems were isothermal bodies that rotate more or less rapidly according to their degree of equatorial-flattening. Galaxies of this type did not seem mysterious. However, the principal reason why ellipticals tended to be neglected in favor of disk galaxies was the great labor involved in obtaining reliable photometry or measuring their mean and random velocities. For practical purposes, and despite the pioneering work of de Vaucouleurs (1948) and Minkowski (1962), both of these types of measurements had to wait until the advent of computer-assisted plate-scanning machines, high-quantum efficiency spectrometers, and computers capable of extracting velocity information from the spectra.
Since 1976, a considerable body of photometric and kinematic data concerning elliptical galaxies has been assembled by astronomers around the world. And just as the merest blade of grass is discovered to be a superbly delicate structure as soon as it is examined under a microscope, so too have the simplest of galaxies proven to be remarkably complex. They are not isothermal and they do not rotate in proportion to their flattening. In this review, it is not possible to say what they are, because theory now lags behind observation in its efforts to coordinate the many observational data available into a self-consistent picture of elliptical galaxies. But the outlines of what may become the standard picture can be descried, and the observational facts can be stated.
The bulges of disk galaxies are only now coming under concerted attack by the observers, but the first results are fascinating. Much hangs on the question of whether bulges really are just small ellipticals, or whether they represent an independent type of beast. For if it can be shown that they form a subspecies of elliptical, this would constitute strong evidence that elliptical galaxies and bulges are the fundamental systems toward which theories of galaxy formation should be directed. But if observation shows that bulges are quite unlike ellipticals, this might indicate that ellipticals are, as Toomre (1977) has argued, formed by the merging of disk galaxies, which then become the basic products of galaxy formation in the primitive Universe.
The range of topics covered in this review has been severely restricted by the availability of space. It has not been possible to include a discussion of the formation and evolution of spheroidal components, or even to give a complete review of the literature concerning their equilibria. Additional information can be found in the reviews of Gott (1977, 1980), Freeman (1975, 1977) and Binney (1980a).
The article is organized as follows. Section 2 covers the basic stellar dynamical concepts. Section 3 is concerned with the radial structure of spheroidal components; Section 3.1 describes theoretical models against which the observations that are reviewed in Section 3.2 may be measured. Section 4 is concerned with the shapes of spheroidal components. Section 4.1 reviews the theoretical situation regarding axisymmetric and triaxial models, and Section 4.2 describes observations relevant to determining the shapes of spheroidal components and the dynamical processes that underlie their figures. Section 5 sums up and looks to the future.