active galaxy


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active galaxy

A galaxy that is emitting unusually large amounts of energy from a very compact central source – hence the alternative name active galactic nucleus or AGN. (A separate category, starburst galaxy, is employed for a galaxy where a high infrared luminosity arises from intense star formation.) The central powerhouse may be observed directly, as in Seyfert galaxies, BL Lac objects, or quasars; in radio galaxies it is the radio-emitting lobes created by beams emanating from the powerhouse that are observed. In general the host galaxies of powerful AGN are large, luminous elliptical galaxies, whereas the hosts of Seyferts are spiral. Although some radio-quiet QSO are found located in disklike hosts, most are found to lie in elliptical galaxies.

Observations of the motions of stars and gas in galaxies such as M87 (see Virgo A) and NGC 4151 (see Seyfert galaxy), in addition to other arguments (see quasar; power-law spectrum), strongly suggest that the energy output is derived from the gravitational potential of a supermassive black hole: the energy arises from an accretion disk of matter spiraling into the black hole. This material could come from the interstellar medium of a spiral galaxy (especially when perturbed by gravitational effects in interacting galaxies), from the tidal disruption of stars near the black hole, or from flows of intergalactic gas on to the central galaxy of a cluster of galaxies, as the gas cools. The X-ray continuum spectrum of many AGN features components of emission that are thought to be reflected off the accretion disk of the black hole, producing a reflection spectrum.

active galaxy

[′ak·tiv ′gal·ək·sē]
(astronomy)
A galaxy whose central region exhibits strong emission activity, from radio to x-ray frequencies, probably as a result of gravitational collapse; this category includes M82 galaxies, Seyfert galaxies, N galaxies, and possibly quasars.
References in periodicals archive ?
The total entropy of the universe and of a single active galaxy sums to zero.
But new observations of a nearby active galaxy called NGC 3783, harnessing the power of the Very Large Telescope Interferometer (VLTI) at ESO's Paranal Observatory in Chile, have given a team of astronomers a surprise.
A super-sized version of the same phenomenon occurs at the center of an active galaxy, where a black hole weighing millions to billions of times our sun's mass can drive outflows extending millions of light-years.
CU-Boulder Professor Michael Shull and Research Associate David Syphers used the Hubble Space Telescope to look at the quasar-the brilliant core of an active galaxy that acted as a "lighthouse" for the observations-to better understand the conditions of the early universe.
Thick clouds of dust and gas surround the black hole in an active galaxy, which can block ultraviolet, optical and low-energy, or soft X-ray, light.
Lamer and his Potsdam colleagues first spotted the massive cluster, dubbed 2XMM J083026+524133, when scrutinizing data from the European Space Agency's XMM-Newton telescope, which captured the cluster's signature in 2001 while imaging a distant, active galaxy.
University of Colorado Boulder Professor Michael Shull and Research Associate David Syphers used the Hubble Space Telescope to look at the quasar-the brilliant core of an active galaxy that acted as a "lighthouse" for the observations-to better understand the conditions of the early universe.
The combined efforts of all these instruments and astronomers gave an unprecedented insight into the core of an active galaxy.
Swift also carries X-ray and ultraviolet/optical telescopes, enabling quick confirmation of new active galaxy candidates.
The investigators hope to detect high-energy neutrinos released in astrophysical events such as the cataclysmic gravitational collapse of a star or the accretion of matter in the massive core of an active galaxy.
This portrait of M106 contains only the inner structure around the halo and nucleus of this Seyfert 2 active galaxy.
And when the galaxy is the nearest active galaxy to us, Centaurus A, which also happens to be one of the strongest and longest studied celestial sources of radio waves, the combination provides a rather unique excitement for astronomers.