Active galactic nucleus
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An active galaxy is a galaxy where a significant fraction of its energy output is not emitted by the normal components of a galaxy: stars, dust and interstellar gas. This energy, depending on the active galaxy type, can be emitted across most of the electromagnetic spectrum, as infrared, radio waves, Ultra Violet, X-ray and gamma rays.
Frequently, the abbreviation AGN (Active Galactic Nuclei) is used, since most 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 106 and 1010 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 emit strongly in the optical, ultra-violet or X-ray bands. 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 consumed 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 Way, and in other nearby galaxies, and also nicely explains why quasars 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.
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[edit] Types of active galaxies
Active galactic nuclei can easily be classified by three parameters: optical variability, radio emission, and spectral line width. Seyfert galaxies are mostly radio-quiet and not so strongly variable as other types of AGN; Seyfert-1's have broad and narrow spectral lines, while Seyfert-2's only have narrow lines. However, some Seyfert-1s exhibit broadened lines that are relatively narrow, and are designated "Narrow Line Seyfert 1s" (NLS1s). Quasars are broad-lined and some of them are variable; they can be further divided between radio-quiet and radio-loud quasars. Radio galaxies have strong radio emission and are not variable. Blazars are highly variable and some of them have strong radio emission; they can be divided between narrow-lined BL Lac objects (some of them show no lines at spectra at all, that's why redshift of many BL Lac's remains unknown) and broad-lined OVV's (Optically Violently Variable quasars).
Seyferts, quasars and blazars are the main types of AGNs that emit high-energy radiation (X-rays), although all AGNs emit X-rays to some extent. Quasars, in particular, are thought to be the most consistently luminous objects in the known universe.
Radio galaxies are active galaxies with strong, extended radio emission, powered by relativistic jets from the active nucleus.
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.
[edit] Optical Spectra
Seyfert 1 spectra are characterized by broad and narrow emission lines. Quasar spectra are qualitatively similar to those of Seyfert 1s.
Seyfert 2 spectra are characterized by narrow emission lines. Their polarized spectra may also show broad lines.
Blazars have highly polarized, variable optical continua. In BL Lac objects, there are no strong emission or absorption lines, while OVV quasars show broad lines.
[edit] X-ray Spectra
The X-ray spectra of AGNs primarily consist of three components: a power-law component which dominates the X-ray emission in the 0.1-10 keV range, a soft component that sometimes dominates below 1 keV, and an Fe K line feature (6.4 keV). The power-law component is generally seen in all X-ray spectra of optically luminous active galaxies, though it may be hidden by absorption, while the iron feature (an indication of reflection) and soft component are only sometimes seen. AGN with jets have an additional component of nuclear X-ray emission originating in the jet: this may dominate in blazars and core-dominated quasars.
