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active galaxyA galaxy that is emitting unusually large amounts of energy from a very compact central source – hence the alternative name active galactic nucleus or AGN. (A separate category, starburst galaxy, is employed for a galaxy where a high infrared luminosity arises from intense star formation.) The central powerhouse may be observed directly, as in Seyfert galaxies, BL Lac objects, or quasars; in radio galaxies it is the radio-emitting lobes created by beams emanating from the powerhouse that are observed. In general the host galaxies of powerful AGN are large, luminous elliptical galaxies, whereas the hosts of Seyferts are spiral. Although some radio-quiet QSO are found located in disklike hosts, most are found to lie in elliptical galaxies.
Observations of the motions of stars and gas in galaxies such as M87 (see Virgo A) and NGC 4151 (see Seyfert galaxy), in addition to other arguments (see quasar; power-law spectrum), strongly suggest that the energy output is derived from the gravitational potential of a supermassive black hole: the energy arises from an accretion disk of matter spiraling into the black hole. This material could come from the interstellar medium of a spiral galaxy (especially when perturbed by gravitational effects in interacting galaxies), from the tidal disruption of stars near the black hole, or from flows of intergalactic gas on to the central galaxy of a cluster of galaxies, as the gas cools. The X-ray continuum spectrum of many AGN features components of emission that are thought to be reflected off the accretion disk of the black hole, producing a reflection spectrum.