Annu. Rev. Astron. Astrophys. 1982. 20: 431-468
Copyright © 1982 by . All rights reserved

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1.1. What is an Active Galaxy?

For our purposes we operationally define an active galaxy to be one in which signs of qualitatively unusual and quantitatively energetic activity (i.e. activity not associated with the evolution of normal stars) are clearly visible and can be connected directly or indirectly to the nucleus. Excluded are simple interacting pairs in which no other signs of abnormalities appear and many emission-line or infrared galaxies, which can be entirely explained as multiple H II regions and/or supernova remnants (French 1980, French & Miller 1981, Weedman 1977a, Weedman et al. 1981). M82 and many other Irr II galaxies may be examples of this last class of interesting but, by our criteria, "inactive" galaxies.

A quantitative definition of what constitutes an active galaxy is perhaps not very useful, since galaxies showing low-level nuclear activity (e.g. Heckman 1980b, Stauffer 1981, Hawley & Phillips 1980) may be in a pre- or post- eruptive stage, and so may yield valuable clues into the origin and evolution of nuclear activity. Such AGNs may be difficult to identify because of extinction (e.g. Abell et al. 1978, Keel 1980, Lawrence & Elvis 1982) or confusion with an HII region (e.g. Wron et al. 1981). The number of weak AGNs increases every time deep searches are made. Heckman (1980b) has shown that fully one third of a complete sample of "normal" galaxies exhibit signs of nuclear activity. It remains to be seen whether all galaxies harbor active nuclei of low luminosity, much like the Milky Way (Balick & Brown 1974, Brown et al. 1981).

Radio and X-ray emission are generally regarded as telltale signs of activity in galaxies, and so we include all classical radio galaxies (e.g. Miley 1980) and X-ray galaxies (e.g. Giacconi 1978, Wilson 1979) in the class of active galaxies. Yet here too ambiguities arise. For example, recent VLA studies of the nuclear regions of "normal" galaxies show a variety of morphologies for the nuclear radio sources, some of which can be interpreted in the context of star formation/supernova remnants, and others that are much more suggestive of nuclear activity (e.g. Condon et al. 1982, van der Hulst et al. 1981). Whether the core radio or X-ray luminosity of a spiral galaxy is an accurate barometer of activity can be problematic.

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