Annu. Rev. Astron. Astrophys. 1984. 22: 185-222 Copyright © 1984 by Annual Reviews. All rights reserved

4.1 A Basic Description of Galaxies

The following is a list of the primary attributes of galaxies and their distributions that a comprehensive theory of formation and evolution should explain.

1. Different morphological types exist. There are spheroidal galaxies, as well as disk galaxies that include a very flat distribution of stars in addition to a spheroid (Morgan 1958, Sandage 1961, de Vaucouleurs et al. 1976). There is a continuous range of disk-to-bulge luminosity ratios (D / B) among disk galaxies, probably melding smoothly into the elliptical galaxies, which have negligible disks. When gas is present, a luminous spiral pattern, related to the sites of star formation in the disk, is usually seen. Most gas-free disk systems (S0s and some early-type spirals) and ellipticals have normally had little star formation within the last several billion years (Searle et al. 1973, Larson & Tinsley 1978, Caldwell 1983).

2. All morphological types are found in both low- and high-density environments. Hubble & Humason (1931), Morgan (1961), and Abell (1965) described the transition to earlier-type galaxies (Es and S0s) in rich clusters, and Oemler (1974) quantified the relationship by identifying characteristic global mixes of E, S0, and spiral galaxies. Dressler (1980b) showed that such global descriptions derive from a tight relation of galaxy morphology to local galaxy density, and this behavior has been shown to extend all the way to the low-density field (Bhavsar 1981, de Souza et al. 1982, Postman & Geller 1984), about five orders of magnitude in space density. The fraction of spiral galaxies decreases as the fraction of S0s and Es increases with local galaxy density, almost independently of global cluster characteristics. This ``morphology-density'' relation is monotonic but extremely slow (roughly logarithmic), so that the low-density field is dominated by 80-90% spirals and the highest-density regions are composed of 80-90% elliptical and S0 galaxies, but all types are represented in all environments.

3. Elliptical galaxies appear to be the simplest systems. To first order they form a one-parameter family in luminosity, presumably proportional to mass. Their sizes, colors (metal abundances?), and characteristic internal velocities (central velocity dispersions) scale monotonically with luminosity (see, for example, Faber & Jackson 1976, Tonry & Davis 1981). There is some evidence of an additional parameter, which could be galaxy ellipticity, surface brightness, or mass-to-light ratio (Terlevich et al. 1981, de Vaucouleurs & Olson 1982, Efstathiou & Fall 1983). The average surface brightness (and true mass density?) of ellipticals rises slowly with increasing brightness, levels off between -19 > M_B > -21, and then begins to fall again (Binggeli et al. 1984). Most elliptical galaxies can be represented by models of oblate isotropic rotators (flattening due to rotation); however, with rising luminosity (M_B < -21), an increasing fraction owe their flatness to anisotropic velocity dispersions (Binney 1981, Illingworth 1983, Davies et al. 1983).

4. S0 galaxies almost always have small D / B (~ 1) and appear to have a ``thick disk'' component in addition to the thin disks found in both S0s and spirals.

5. Spirals are classified by their D / B and by the detailed form of their spiral pattern (e.g. openness of the arms, arm thickness, contrast of the arms to the underlying disk; Sandage 1961). Their forms do not correlate well with luminosity, but the spiral pattern and gas content seem related to D / B (Strom 1980). At least a two-parameter family is indicated (Brosche 1973, Whitmore 1984).

6. More-detailed morphological and kinematical structure is reviewed by Kormendy (1982). These numerous constraints are beyond the scope of this review and are more detailed than the general models that are discussed here.

7. Along with these characteristics, it is important to remember that the dominant structure in some or all types may be a massive, unseen halo, and interactions among halos may have a critical effect on the luminous matter within them.