A galaxy that is extremely luminous at radio
wavelengths. It is usually a giant elliptical
– the largest galaxy in a cluster. The radiation emission may come
only from the nucleus, but more typically also comes from a pair of more
or less symmetric lobes stretching as far as a million light-years either
side of the nucleus. Many radio galaxies also show emission from jets
connecting the nucleus to these lobes. Whatever produces the radio emission
must have a long "memory," preserving its direction over millions of years.
The only reasonable explanation for the central energy source, as in the
case of all active galactic nuclei, is a supermassive
black hole, that squirts matter out along its spin axis and so delivers
a steady supply of energy to the lobes. Among the brightest radio galaxies
are Centaurus A, Cygnus
A, and Virgo A (see M87).
| Radio galaxy 3C305