Astrophysics (index)about

active galactic nucleus

(central region of a galaxy with extremely high luminosity)

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-ray bands. A galaxy hosting one is called an active galaxy. X-rays are useful for identifying AGNs because virtually all produce them and they penetrate the surrounding galaxy, which have no X-ray sources strong enough to lead to confusion.

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.

AGNs are sometimes classified using a classification that originated with Seyfert galaxies, which harbor AGNs.

(galaxies,quasars,EMR,galaxy nucleus)

Referenced by:
accretion rate
active galaxy
Arakelian Catalog (Ark)
broad emission line region
black hole accretion rate (BHAR)
Chandra Deep Field South (CDFS)
Compton reflection
cosmic X-ray background (CXB)
direct collapse black hole (DCBH)
Fanaroff-Riley classification
galaxy merger
gas streamer
gravitational collapse
gravitational lensing
hyperluminous infrared galaxy (HLIRG)
hot DOG
ionizing radiation
jet current
luminous infrared galaxy (LIRG)
Whirlpool Galaxy (M51a)
Meier paradox
N galaxy
obscured fraction-luminosity relation
Poynting vector (S)
quasar (QSO)
quenched galaxy
Rosat Bright Survey (RBS)
relativistic beaming
retrograde accretion
Seyfert galaxy
supermassive black hole (SMBH)
submillimeter galaxy (SMG)
superluminal motion
tidal disruption event (TDE)
Thomson optical depth
ultraluminous infrared galaxy (ULIRG)
ultraluminous X-ray source (ULX)
Wide-field Infrared Survey Explorer (WISE)
X-ray luminosity function (XLF)
X-ray source (RS)