active galactic nucleus
(AGN)
(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.
(galaxies,quasars,EMR,galaxy nucleus)
Further reading:
http://en.wikipedia.org/wiki/Active_galactic_nucleus
Referenced by pages:
accretion rate
ACIS
active galaxy
Arakelian Catalog (Ark)
broad emission line region
black hole accretion rate (BHAR)
bipolar outflow
blazar
broad-line region (BLR)
Chandra Deep Field South (CDFS)
Cloudy
Compton reflection
cosmic X-ray background (CXB)
direct collapse black hole (DCBH)
extragalactic astronomy
star formation feedback
Fanaroff-Riley classification
galactic astronomy
galactic wind
galaxy merger
gas streamer
gravitational collapse
gravitational lensing
high-energy astrophysics (HEA)
hyperluminous infrared galaxy (HLIRG)
hot DOG
ionizing radiation
jet current
K-line
LINER
luminous infrared galaxy (LIRG)
LaSilla-QUEST Variability Survey (LSQ)
Whirlpool Galaxy (M51a)
maser
Meier paradox
microwave
MOJAVE
N galaxy
obscured fraction-luminosity relation
plasma astrophysics
Poynting vector (S)
quasar (QSO)
quenched galaxy
radiation pressure
Rosat Bright Survey (RBS)
relativistic beaming
relic
retrograde accretion
radio galaxy (RG)
radio source (RS)
Seyfert galaxy
supermassive black hole (SMBH)
spectropolarimetry
superluminal motion
tidal disruption event (TDE)
thermal dust emission
Thomson optical depth (τT)
3C 273
transient astronomy
ultraluminous infrared galaxy (ULIRG)
ultraluminous X-ray source (ULX)
ultraviolet astronomy
Wide-field Infrared Survey Explorer (WISE)
XBONG
X-ray luminosity function (XLF)
X-ray
X-ray source
Index