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 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 10⁹ 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 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.

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.

Types of active galaxies

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.