Active Galaxy — An active galaxy is a galaxy where a significant fraction of the energy output is not emitted from normal stellar populations or 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 of the active galaxies emit most of their radiation from a narrow region in their nucleus. This is not true for some of them: for example, radio galaxies usually emit radio waves from big lobes around the main galaxy body, and infrared starburst galaxies can scatter their emission around their body.
Anyway, most of high-energy emissions appears to be from the galaxy’s nucleus, and the term AGN is mostly used for this kind of active galaxies.
Types of active galaxies
Seyfert’s, quasars and blazars are the main types of high-energy AGNs. Quasars, in particular, are thought to be the most luminous objects in the known universe.
They are linked by a theory eloquently called the Unified Theory of Active Galaxies, which states that they are the same object seen from different directions. Most astronomers think that this kind of objects are powered by a supermassive black hole.
Radio galaxies are an etherogeneous group of radio-emitting objects. Most of them have huge symmetrical lobes from which the most of radio waves come. Some of them show a jet (the most famous example being the giant galaxy M87 in the Virgo cluster) coming directly from the nucleus and going to the lobes. The nucleus activity suggests a black hole explanation also for this case.
Starburst galaxies emit a high percentage of their light (sometimes as high as 99%) as infrared light. It is thought that these galaxies are experiencing a period of massive star formation, and the copious amount of radiation from the young stars is absorbed by molecular clouds, which then re-emit it in the infrared domain.