Active galactic nucleus

An active galactic nucleus (AGN) is a compact region at the center of a galaxy that has a much higher than normal luminosity over at least some portion – and possibly all – of the electromagnetic spectrum, with characteristics indicating that the excess luminosity is not produced by stars. Such excess non-stellar emission has been observed in the radio, microwaves, infrared, optical, ultra-violet, X-ray and gamma ray wavebands. A galaxy hosting an AGN is called an active galaxy. The radiation from an AGN is believed to be a result of accretion of matter by a supermassive black hole at the center of its host galaxy. AGN are the most luminous persistent sources of electromagnetic radiation in the universe, and as such can be used as a means of discovering distant objects; their evolution as a function of cosmic time also puts constraints on models of the cosmos. The observed characteristics of an AGN depend on several properties such as the mass of the central black hole, the rate of gas accretion onto the black hole, the orientation of the accretion disk, the degree of obscuration of the nucleus by dust, and whether jets are present. Numerous subclasses of AGN have been defined based on their observed characteristics; the most powerful AGN are classified as quasars.

For a long time it has been argued that an AGN must be powered by accretion of mass onto massive black holes (10⁶ to 10¹⁰ times the Solar mass). AGN are both compact and persistently extremely luminous. Accretion can potentially give very efficient conversion of potential and kinetic energy to radiation, and a massive black hole has a high Eddington luminosity, and as a result, it can provide the observed high persistent luminosity. Supermassive black holes are now believed to exist in the centres of most if not all massive galaxies since the mass of the black hole correlates well with the velocity dispersion of the galactic bulge (the M-sigma relation) or with bulge luminosity. Thus AGN-like characteristics are expected whenever a supply of material for accretion comes within the sphere of influence of the central black hole.

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