An active galaxy is a galaxy where a significant fraction of the 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, 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 model is that the energy is generated by matter falling onto a supermassive black hole of between 1 million and 1 billion solar masses. 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 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.
Seyferts, quasars and blazars are the main types of AGNs that emit high-energy radiation (X-rays). Quasars, in particular, are thought to be the most consistently luminous objects in the known universe.
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 cluster) 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.
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.Types of active galaxies