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.
Such atypical emission has been observed in radio,
infrared, visible light, X-ray, and gamma-ray
bands. A galaxy hosting an AGN 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 other X-ray sources strong enough to create any doubt.
The radiation is assumed to be due to accretion of mass
by a supermassive black hole at the center of the galaxy. Often associated with
an AGN 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:
- Limitation to the rate that the black hole accrete matter.
- Surrounding cold gas heated so it expands, slowing star formation.
- Surrounding cold gas compressed, increasing star formation, possibly leading to a starburst.
The outflow creates a region of plasma whose temperature 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.
Some classes of AGN:
- NLAGN (narrow-line AGN) versus BLAGN (broad-line AGN): the width of emission lines which is taken as Doppler broadening, indicating radial velocity of the source of the spectral line, i.e., movement of the gas/plasma.
- LLAGN (low-luminosity AGN): a lower X-ray emission than a "normal" AGN, though it is possible the LLAGNs are the more frequent.
Referenced by pages:
Arakelian Catalog (Ark)
broad emission line region
black hole accretion rate (BHAR)
broad-line region (BLR)
Chandra Deep Field South (CDFS)
cosmic X-ray background (CXB)
direct collapse black hole (DCBH)
hyperluminous infrared galaxy (HLIRG)
luminous infrared galaxy (LIRG)
LaSilla-QUEST Variability Survey (LSQ)
Whirlpool Galaxy (M51a)
obscured fraction-luminosity relation
Poynting vector (S)
Rosat Bright Survey (RBS)
radio source (RS)
supermassive black hole (SMBH)
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)