active galactic nucleus

An active galactic nucleus (AGN) is a central region of a galaxy with very high luminosity. Such atypical emission has been observed in radio, infrared, visible light, X-ray, and gamma-ray bands. A galaxy hosting an AGN is called an active galaxy. X-rays are useful for identifying AGNs because virtually all produce them and they penetrate the surrounding galaxy, which have no other X-ray sources strong enough to create any doubt. The radiation is assumed to be due to accretion of mass by a supermassive black hole (SMBH) at the center of the galaxy. Often associated with an AGN is a relativistic jet. The radiation as well as matter spun from an accretion disk is called the AGN outflow or AGN feedback (see star formation feedback) for which several possible effects are theorized:

• Limitation to the rate that the black hole accrete matter.
• Surrounding cold gas heated so it expands, slowing star formation.
• Surrounding cold gas compressed, increasing star formation, possibly leading to a starburst. Note that subsequent to such a burst, cold gas may be depleted and star formation minimal.

The outflow creates a region of plasma whose temperature causes it to have a lower density than the surrounding gas of the same pressure. The term AGN bubble is used for the region. The plasma can give off X-rays. The extremely high luminosity of some AGNs (in theory, higher than could be sustained) has led to theories of pulses. Some observed periodicity seems unlikely to be produced by a black hole, suggesting involvement of a pulsar. AGNs are sometimes classified using a classification that originated with Seyfert galaxies, which harbor AGNs. Some classes of AGN:

• NLAGN (narrow-line AGN, Type 2 AGN) versus BLAGN (broad-line AGN, Type 1 AGN): based upon the width of emission lines which is taken as Doppler broadening, indicating radial velocity of the source of the spectral line, i.e., motion of the gas/plasma.
• LLAGN (low-luminosity AGN): a lower X-ray emission than a typical AGN, though it is possible the LLAGNs are the more frequent.

Some AGNs (termed changing-look AGNs) change from one such type or classification to anther as time passes.

I describe AGN above as an observational term, but I believe that it is often used for the presumed phenomenon, a central SMBH undergoing accretion, thus the term is used even if the phenomenon is hidden, or looks different, e.g., one could say "a quasar is an AGN".