= Active galactic nucleus
{title2=AGN}
{wiki}
= Active galactic nuclei
{synonym}
An active galactic nucleus is the compact luminous centre of a <galaxy> powered mainly by <black-hole accretion> onto a <supermassive black hole>. Its <radiation> can dominate the host at some wavelengths even though the emitting region is small compared with the <galaxy>. Observers identify activity using combinations of <emission lines>, a compact <radio wave> source, energetic <X-rays> and brightness changes. Different components respond on different timescales: <reverberation mapping> measures the delay between changes in the central radiation and the response of surrounding line-emitting <gas>, giving a light-travel estimate of its distance. Combining that distance with the <Doppler broadening> of the <emission lines> constrains the central <mass> if the emitting <gas> is gravitationally bound. Activity can be episodic; the existence of a central <black hole> alone does not imply a luminous nucleus.<Coevolution (Or Not) of Supermassive Black Holes and Host Galaxies by John Kormendy and Luis C. Ho>[Section 3.4 on reverberation-based masses and section 7.1 on optical, radio and variable X-ray signatures of nuclear accretion]{ref}
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