An Active Galactic Nucleus (AGN) is a central region of a Galaxy with very high Luminosity. Excess Emission has been observed in Radio, Infrared, Visible Light, X-ray, and Gamma Rays bands. A galaxy hosting one is called an Active Galaxy.
The radiation is assumed to be due to Accretion of mass by a Supermassive Black Hole. Often associated with a nucleus is a Relativistic Jet. The radiation as well as matter spun from an Accretion Disk is called the AGN Outflow or AGN Feedback (see Star Formation Feedback) for which several possible effects are theorized:
The outflow creates a region of Plasma whose heat causes it to have a lower density than the surrounding gas of the same pressure. The term AGN Bubble is used for the region. The plasma can give off X-Rays.
The extremely high luminosity of some AGNs (in theory, higher than could be sustained) has led to theories of pulses.
Some observed periodicity seems unlikely to be produced by a black hole, suggesting involvement of a Pulsar.
Arakelian Catalog (Ark)
Chandra Deep Field South (CDFS)
Cosmic X-Ray Background (CXB)
Hyperluminous Infrared Galaxy (HLIRG)
Luminous Infrared Galaxy (LIRG)
Whirlpool Galaxy (M51a)
Rosat Bright Survey (RBS)
Supermassive Black Hole (SMBH)
Submillimeter Galaxy (SMG)
Tidal Disruption Event (TDE)
Ultraluminous Infrared Galaxy (ULIRG)
Ultraluminous X-Ray Source (ULX)
Wide-Field Infrared Survey Explorer (WISE)