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This energy, depending on the active galaxy type, can be emitted across most of the electromagnetic spectrum, as infrared, radio waves, UV, X-ray and gamma rays.
Frequently, the abbreviation AGN (Active Galactic Nuclei) is used, since all active galaxies appear to be powered by a compact region in the galactic centre. Some of these compact regions emit jets of matter that can extend for very large distances, powering extended structures (such as radio galaxies and radio-loud quasars). But in all cases the active nucleus or central engine is the fundamental source of energy.
The standard theoretical model is that the energy is generated by matter falling onto a supermassive black hole of between 1 million and 1 billion solar masses. As the material falls into the black hole, angular momentum causes the material to flatten into an accretion disk. Frictional heating causes the infalling material to turn into plasma, and this charged moving material produces a strong magnetic field via the alpha mechanism . Frequently, one observes jets emanating from the accretion disk, although the mechanism of formation of those jets is poorly understood. The accretion mechanism is highly efficient at turning matter into energy, and can convert almost 50% of the mass-energy of an object into energy as compared with only a few percent with nuclear fusion.
It is believed that when the black hole has eaten all of the gas and dust in its neighborhood that the active galactic nucleus ceases to emit large amounts of radiation and becomes a normal galaxy. This model is supported by what appears to be a quiet supermassive black hole in the center of the Milky WayThis article is about the galaxy called the Milky Way. For the candy bar of the same name, see Milky Way candy bar. The Milky Way (a translation of the Latin Via Lactea in turn derived from the Greek Galaxia gala, galactos means "milk")) is a hazy band of, and in other nearby galaxies, and also nicely explains why quasarA quasar (from quasi-stellar radio source is an astronomical object that looks like a star in optical telescopes (i. it is a point source), and has a very high redshift. The general consensus is that this high redshift is cosmological, the result of Hubbls appear to have been much more common in the early universe , when more fuel was available.
This model also explains the different types of active galactic nuclei, which are believed to all be due to the same type of source, but can appear quite different depending on the angle the source makes to the earth, and the amount of gas and dust available to be fed into the black hole.
Radio galaxies are a heterogeneous group of radio-emitting objects. Most of them have huge symmetrical lobes from which the greater part of the radio emission comes. Some of them show a jet or jets (the most famous example being the giant galaxy M87 in the Virgo clusterLenticular Galaxy M84; the elongated image of NGC 4388 (an active spiral galaxy) is in the lower left corner. The Virgo cluster is a cluster of galaxies, approximately 15 to 22 Mpc distant, comprising approximately 1300 (and possibly up to 2000) member ga) coming directly from the nucleus and going to the lobes. The jets are believed to be the visible manifestations of the beam of high-energy particles that power the lobes. The radio emission is synchrotron radiationSynchrotron radiation refers to electromagnetic radiation similar to cyclotron radiation, but generated by electrons moving at highly relativistic speeds. Synchrotron radiation can be generated by astronomical structures and motions. See also synchrotron, implying that the radio lobes and jets contain relativistic electrons and magnetic fields.
Some of the different types of active galaxy are linked by Unified models in which they are really the same class of object seen at different viewing angles, with relativistic beaming and dust obscuration causing the observational differences. The two main unified models link the different classes of Seyferts and radio galaxies, quasars and blazars.
See also: Quasar, Seyfert galaxy, Blazar, Black hole.
Galaxies Active galaxies