Active galactic nuclei are small regions in the center of galaxies that emit large amounts of radiation due to matter accumulating on supermassive black holes. They are divided into radio-noisy and radio-silent AGNs, with the former possessing relativistic jets. Quasars are a subset of AGNs and act as windows into the early universe.
An active galactic nucleus is a small region, between about 1 and 100 light-years in diameter, in the center of a galaxy that emits prodigious amounts of radiation over all or part of the electromagnetic spectrum, from the shortest gamma rays to the longest radio waves. Active galactic nuclei are among the most energetic phenomena in the universe, even beating supernovae in terms of luminosity.
Active galactic nuclei are fueled by matter that accumulates on supermassive black holes, with a mass between 106 and 1010 times that of the Sun. These black holes are generally about one light-year in diameter. The infalling matter forms into an accretion disk, with gas particles moving and colliding with each other at near the speed of light. This releases huge amounts of energy, often in the form of relativistic jets that project perpendicular to the accretion disk. Depending on whether or not these jets are pointing towards Earth, an active galactic nucleus could have very large luminosity or very little luminosity.
Active galactic nuclei are far more common in older and therefore more distant galaxies. Quasars, a subset of active galactic nuclei, are among the most distant observed objects in the universe, with observed distances equal to or greater than 13 billion light-years. These quasars act as windows into the early universe. While all massive galaxies are thought to have central supermassive black holes that could theoretically fuel active galactic nuclei, in practice only older galaxies appear to host them. This is probably due to the fact that the matter in the central regions of the galaxies had not yet fallen into the hole and was therefore in greater quantities. The most powerful active galactic nuclei consume about one solar mass of material each year.
Active galactic nuclei are generally divided into radio-noisy and radio-silent AGNs. Radio-quiet active galactic nuclei tend to lack relativistic jets, releasing heat and light through accretion of matter but without the projections. Radio active galactic nuclei possess relativistic jets, and in the brightest objects, blazars, these jets are found very close to the line of sight.
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