Annu. Rev. Astron. Astrophys. 1984. 22: 471-506 Copyright © 1984 by Annual Reviews. All rights reserved

1. INTRODUCTION

It is now 20 years since active galactic nuclei (AGNs) became widely acknowledged as an important astrophysical phenomenon (33, 109). Over the entire subsequent period, one of the few statements to command general agreement has been that the power supply is primarily gravitational: the whole bestiary of models involving dense star clusters, supermassive stars, or black holes at least have this feature in common. Systems dependent on gravitational energy have something else in common: they all undergo an inexorable runaway as the central potential well gets deeper and deeper. According to conventional physics, the almost inevitable endpoint of any dense star cluster or supermassive star will be the collapse of a large fraction of its total mass to a black hole. This is the "bottom line" of Figure 1. Such arguments suggest that massive black holes should exist in the nuclei of all galaxies that have ever experienced a violently active phase. Furthermore, physical processes involving black holes offer a more efficient power supply than any of the "precursor" objects depicted in Figure 1. So massive black holes may not merely be the defunct remnants of violent activity; they may also participate in its most spectacular manifestations.

Considerations such as these have shifted the emphasis of theoretical work away from dense star clusters and supermassive stars and have motivated fuller (or at least less perfunctory) investigations of how black holes might generate the power in quasars, radio galaxies, and related objects. All of the evolutionary tracks in Figure 1 deserve more study: none can be dismissed as irrelevant to the AGN phenomenon. The present review is nevertheless focused on black hole models. Moreover, its scope is even more restricted: I am primarily concerned here with what goes on close to the black hole - in the region where the gravitational potential is not merely "(1/r)," but where intrinsically relativistic features can also be significant. Although this is where the power output is concentrated, many conspicuous manifestations of AGNs - the emission lines, the radio components, etc. - involve some reprocessing of this energy on larger scales. For this reason (and also because of space limitations), little is said here about phenomenology: I merely discuss some physical processes and simple idealized models that have been advanced as ingredients of AGNs.

Figure 1. Schematic diagram [reproduced from Rees (106)] showing possible routes for runaway evolution in active galactic nuclei.