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

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Fifty-eight years ago Hubble expanded the vistas of astronomy from the Milky Way into what appeared to be a serene Universe. Forty years later the recognition of quasars (QSOs) permanently upset this notion of extragalactic serenity. With time has come the realization that every large galaxy may have erupted in QSO-like activity, perhaps repeatedly. The rich history of the field of active galaxies can be found in Weedman's (1976) fascinating review.

Active galactic nuclei (AGN) and their more spectacular counterparts, the QSOs, have received a tremendous amount of attention from both the observational and theoretical front. This attention has centered on the phenomenology of AGNs and QSOs (e.g. studies of the nonthermal radio emission, the bright, broad emission lines) leaving largely unexplored the question of how and why certain galaxies become "active." While there has been a wealth of clever speculation about the nature of the "machine" that powers the activity, no single idea has gained universal acceptance. To discriminate among such plethoric ideas as massive black holes and their accompanying accretion disks or spheres, spinars, magnetoids, dense star clusters, giant stars, and star-antistar collisions (Rees 1977, 1978a, b, McCray 1979, Alven 1979, Carter 1979, Fabian 1979, Lu et al. 1977, Ozernoy & Reinhardt 1978, Vil'koviskij 1978, Hills 1978, McMillan et al. 1981, Shields & Wheeler 1978, Edwards 1980, Pacini & Salvati 1978, 1981, Jones & Racine 1980, and many earlier papers referenced therein) may require knowledge about how nuclear activity is triggered. Accordingly, in this article we concentrate on clues to the origin and evolution of nuclear activity and leave aside questions of nuclear phenomenology and taxonomy.

Even though our approach emphasizes observations, active galaxies are inherently complex in nature, and ours is an interpretive rather than purely descriptive problem. Consequently, theoretical concepts are crucial to our discussion. This paper, then, is a collection of interpretive ideas by extragalactic archeologists searching through the habitats of the most unruly residents of the Universe. Such a search is particularly challenging because AGNs have the power to obliterate, alter, and obscure the clues that we seek, especially in their immediate surroundings.

The number of relevant papers published in the past five years alone exceeds three hundred. In the interest of conciseness and timeliness we emphasize only the recent work; earlier references can be located from those cited here. For obvious reasons of spatial scale, our search emphasizes the closest active galaxies, primarily Seyfert, radio, and X-ray galaxies and, as a control sample, "normal" or undisturbed galaxies.

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