The powerful radio galaxies and quasars have been recognized largely as a result of the optical identification of radio sources that have been catalogued in various radio surveys. By contrast, sources at the low end of the radio luminosity function are observed primarily by measuring the radio emission from known optical objects. The most extensive surveys are those of Sadler (1984), Kotanyi (1980), Hummel (1980), and Dressel and Condon (1978). These so-called "normal galaxies" typically have a radio luminosity between 1036 and 1039 erg s-1. They are primarily ellipticals, but some spirals are also detected as weak radio sources (e.g., Ekers 1978, 1981, Sadler 1984, Preuss 1987).
The observed radio emission may come from an extended component comparable in size to the optical image or from a bright compact nucleus. Although the luminosities of the weak radio nuclei are some million times less than those found in radio galaxies and quasars, the surface brightness, magnetic field strength, structure, and time scale of observed intensity variations are all comparable to those of their more luminous counterparts discussed in Section 13.3.