All galaxies and quasars appear to be sources of radio emission at some level. Normal spiral galaxies such as our own galactic system are near the low end of the radio luminosity function and have radio luminosities near 1037 erg s-1. Some Seyfert galaxies, starburst galaxies, and the nuclei of active elliptical galaxies are 100 to 1000 times more luminous. Radio galaxies and some quasars are powerful radio sources at the high end of the luminosity function with luminosities up to 1045 erg s-1.
For the more powerful sources, the radio emission often comes from regions well removed from the associated optical object, often hundreds of kiloparsecs or even megaparsecs away. In other cases, however, particularly in active galactic nuclei (AGN) or quasars, much of the radio emission comes from an extremely small region with measured dimensions of only a few parsecs. The form of the radio-frequency spectra implies that the radio emission is nonthermal in origin; it is presumed to be synchrotron radiation from ultra-relativistic electrons with energies of typically about 1 GeV moving in weak magnetic fields of about 10-4 gauss (see Section 1.1).
The extended radio sources constitute the largest known physical structures in the universe. Their energy content is very large, up to 1060 erg or more. The origin of this energy and the manner in which it is converted into relativistic particles and magnetic fields has remained one of the most challenging problems of modern astrophysics. High-resolution radio images generally show a very compact component which is coincident with an active galactic nucleus or quasar, and which is thought to reflect the "central engine." Long thin "jets" extend away from the compact central core toward the outer radio lobes. Often the jets end up at hot spots (Figure 13.1).