active galactic nucleus (AGN)
Central energyproducing region in some GALAXIES. AGNs are distinct in
having substantial portions of their energy output coming
from processes that are not associated with normal stars
and their evolution. The observed guises of this energy
release define the various types of active nuclei.
At the lowest power levels are LINERS(Low Ionization
Nuclear Emission-line Regions), generally recognized
only by the ratios of fairly weak EMISSION LINES; not all
LINERS are genuine active nuclei. Activity characterized by strong, broad emission lines occurs in SEYFERT
GALAXIES, most of which are spirals; Seyfert types 1 and
2 have different patterns of line width. Seyfert galaxies
also show strong X-ray emission and, often, far-infrared
radiation. RADIO GALAXIESare most notable for their
strong radio emission, usually from a pair of lobes symmetrically placed about the galaxy, often accompanied
by JETS and radio emission from the nucleus itself. This
activity may have little or no trace in the optical region,
although some radio galaxies do have spectacular optical
emission lines similar to those of both types of Seyfert
galaxy. Higher-luminosity objects are QUASISTELLAR
OBJECTS(QSOs), which are known as QUASARS(quasistellar radio sources) if they exhibit strong radio emission. These objects are so bright that the surrounding
galaxy can be lost for ordinary observations in the light
of the nucleus. Members of another class, BL LACERTAE
OBJECTS, show featureless spectra and rapid variability,
suggesting that they are radio galaxies or quasars seen
along the direction of a relativistic jet, the radiation of
which is strongly beamed along its motion. These categories share features of strong X-ray emission, large
velocities for the gas seen in emission lines, and a very
small emitting region as seen from variability. Many
show variation in the ultraviolet and X-ray bands on
scales of hours to days, implying that the radiation is
emitted in a region with light-crossing time no longer
than these times.
The most popular model for energy production in all
these kinds of active galactic nuclei involves material
around a supermassive BLACK HOLE(of millions to a few
thousand million solar masses). The power is released
during ACCRETION, likely in an ACCRETION DISK, while
jets may be a natural by-product of the disk geometry
and magnetic fields.
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