|Annu. Rev. Astron. Astrophys. 1982. 20:
Copyright © 1982 by . All rights reserved
1.2. The Scope of the Review
Our search for clues to the origin and evolution of activity in galaxies leads in a variety of directions. First we consider the environment of presently active galaxies in order to understand the possible effects of cluster or group membership on the one hand, and nearby interacting companions on the other.
Then we consider the relationships between galactic activity and the internal properties of galaxies, such as stellar content, mass, dynamics, structure, and optical luminosity. We also briefly examine the distributions, motions, and physical and chemical properties of the gas and dust in active galaxies for historical clues.
Several problems must be borne in mind. One is that distinctions between active and normal galaxies often turn out to involve subtleties or questions of degree. Another is the Malmquist bias; the most active galaxies are also frequently the brightest and rarest. A third problem is that many components of a galaxy, and especially the gas and dust, are susceptible to distortion by both the causes and the effects of activity, so interpretive ambiguity or controversy concerning causal relationships is frequent. Finally, as mentioned before, the problem at hand is interpretive rather than descriptive, and therefore the discussion is constrained by the limits of present theoretical knowledge and the availability of models.