Specifically they are in orbit around the centre of the galaxy. Thus, they have angular momentum and as they fall into the potential well of the massive object, instead of falling straight onto the central body, they spiral downwards. This causes the material to build up in a disc around the object. The more mass contained by the object the deeper its potential well and the faster the material in the accretion disc will swirl around it. Thus, the more heating the disc will suffer and the more energy it will release in the form of electromagnetic radiation.
Accretion discs are, primarily, found around protostars and black holes.
A galactic nucleus in which some kind of extraordinary activity is liberating vast amounts of energy in the form of electromagnetic radiation. Sometimes the energy released by the nucleus is so great that it outshines all the other stars in the galaxy combined. Most astronomers believe that, at the centre of an active galactic nuclei, lies a black hole . A minority believe that AGN can be explained as starbursts and multiple supernovae. A galaxy in which some kind of unusual, perhaps violent, phenomenon is releasing energy.