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An active galaxy is a galaxy whose central region has a very high luminosity, this central region termed an active galactic nucleus (AGN). Such excess emission has been observed in radio, infrared, visible light, X-ray, and gamma rays bands. The nucleus's high luminosity is presumed to be due to accretion of mass by a central supermassive black hole (SMBH). Often associated with such a nucleus is a relativistic jet.
The term active galaxy is used generally for any galaxy with such an accreting central SMBH. Their appearance to us depends upon their distance, the galaxies' other structure, and the direction from which we are viewing them. Among the terms based upon these various appearances are Seyfert galaxy, LINER galaxy, radio galaxy, quasar, and blazar. Also, some luminous infrared galaxies (LIRGs), ultraluminous infrared galaxies (ULIRGs), and X-ray sources (these being various terms for bright sources with some specific characteristics) are active galaxies.