Active Galactic Nuclei

Abstract

Active galactic nuclei (AGN) represent an extreme stage in the life cycle of a galaxy. For a relatively short period of time (∼ 108 years or less), a region less than a parsec across at the center of a galaxy produces tremendous amounts of energy, often outshining the rest of the galaxy by orders of magnitude. The most luminous of these objects are the most powerful, continuously emitting sources in the universe. The observational characteristics of AGN are reviewed, as well as how these properties are used to sort them into different classes. The evidence that supports the current paradigm, under which the central source powering AGN is a supermassive black hole, is discussed. While these are found in virtually all bright galaxies, in AGN the black holes are actively accreting matter, most likely as a result of an increased matter density in their immediate environs. A variety of accretion models are considered, particularly disk and Bondi accretion, along with the mechanisms by which material is carried inward and angular momentum is dissipated. Photoionization models for the broad and narrow emission line regions are reviewed in detail, including the physical conditions that prevail in each and their location relative to the central black hole. The evidence for unified models is presented. Under unified models, different types of AGN are related by means of viewing angle. A key aspect of these models is the presence of large-scale obscuring regions within the active nucleus. The phenomenology of nuclear absorption is discussed, both within the torus as well as in lines, along with present models for these obscuring regions. Also discussed are generation mechanisms for and the physics of relativistic jets, which are present in 10–20% of AGN. Finally, the entire picture is brought together by discussing the evolution of AGN.