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An active galactic nucleus (AGN) is a compact region at the centre of a galaxy which has a much higher than normal luminosity over some or all of the electromagnetic spectrum (in the radio, infrared, optical, ultra-violet, X-ray and/or gamma ray wavebands). A galaxy hosting an AGN is called an active galaxy. The radiation from AGN is believed to be a result of accretion on to the supermassive black hole at the centre of the 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 provides constraints on cosmological models. For a long time it has been argued (e.g. Lynden-Bell 1969) that AGN must be powered by accretion onto massive black holes (with masses between 106 and 1010 times that of the Sun). 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, so that it can provide the observed high persistent luminosity. Central supermassive black holes are now believed to exist in the centres of all massive galaxies the mass of the black hole correlates well with the velocity dispersion or luminosity of the bulge of the galaxy (e.g. Marconi & Hunt 2003). Thus AGN-like characteristics are expected whenever a supply of material for accretion comes within the sphere of influence of the central black hole. In the standard model of AGN, cold material close to the central black hole forms an accretion disc. Dissipative processes in the accretion disc transport matter inwards and angular momentum outwards, while causing the accretion disc to heat up. The expected spectrum of an accretion disc around a supermassive black hole peaks in the optical-ultraviolet waveband; in addition, a corona of hot material forms above the accretion disc and can inverse-Compton scatter photons up to X-ray energies. The radiation from the accretion disc excites cold atomic material close to the black hole and this radiates via emission lines. A large fraction of the AGN's primary output may be obscured by interstellar gas and dust close to the accretion disc, but (in a steady-state situation) this will be re-radiated at some other waveband, most likely the infrared. At least some accretion discs produce jets, twin highly collimated and fast outflows that emerge from close to the disc (the direction of the jet ejection must be determined either by the angular momentum axis of the disc or the spin axis of the black hole). The jet production mechanism and indeed the jet composition on very small scales are not known at present, as observations cannot distinguish between the various theoretical models that exist. The jets have the most obvious observational effects in the radio waveband, where Very Long Baseline Interferometry can be used to study the synchrotron radiation they emit down to sub-parsec scales. However, they radiate in all wavebands from the radio through to the gamma-ray via the synchrotron and inverse-Compton process, and so AGN with jets have a second potential source of any observed continuum radiation.
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