Radio Active Nuclei

For the purpose of this survey we shall use the term radio active nucleus to refer to compact radio sources which are found in galactic nuclei and which show activity either in the form of flux density variations or outward flow of material from the nucleus. There is great similarity between the observed properties of radio active nuclei with those of radio active quasars, and it is widely supposed that the quasar phenomena represents an extreme case of an active galaxy. In this paper however, we shall concentrate on compact radio sources found in the nuclei of identifiable galaxies, and quasars are discussed only to the extent that they help to understand the origin and evolution of radio active galactic nuclei. Since radio active nuclei are typically in extent, they can be mapped only with VLBI techniques. Detailed maps are available for only a few of the strongest nuclei, but a much larger number can be identified from conventional interferometry with a resolution ~ 1”. In the few radio galaxies where most of the observed radio flux comes from the active nucleus, activity is observed directly on a time scale of months or even weeks. In others, it may be inferred from arguments based on synchrotron lifetime.