radio galaxy

A galaxy that is an intense source of radio emission. About one galaxy in a million is a radio galaxy. The radio emission is synchrotron radiation from electrons travelling at speeds close to that of light. In Cygnus A, often regarded as the prototype radio galaxy, two huge clouds of radio emission, disposed symmetrically on each side of a disturbed elliptical galaxy, span more than three million light years. It seems unlikely that the huge energy output can be generated by normal nuclear reactions in stars. Mechanisms in which black holes act as a kind of "central engine" have been suggested. Radio galaxies are closely related to quasars, many of which have similar radio properties.