Annu. Rev. Astron. Astrophys. 1994. 32: 115-52 Copyright © 1994 by Annual Reviews. All rights reserved

Radio Continuum Emission

The radio radiation from galaxies generally has two spatial components: 1. that within a nuclear or central region where there may also be various levels of substructure, e.g. jets and knots, and 2. a more extended region, where again there may be structure. One or the other component may be very weak or absent (to the current levels of detection). The literature on radio radiation from normal galaxies is extensive; reviews are given by Condon (1992) and by Hummel (1990).

There are two basic mechanisms responsible for the radio continuum radiation in normal galaxies: synchrotron radiation arising from relativistic electrons accelerated by supernova remnants, and free-free radiation primarily from H II regions. A third origin due to dust heated by starlight is significant only at wavelengths < 1 mm. Because this review is intended to discuss typical galaxies, we omit consideration of ``radio galaxies,'' i.e., radio-loud systems having radio luminosities > 10³³ watts (e.g., Virgo A, Cygnus A).

Most normal galaxies are, at best, weak radio sources, and the statistics concerning their radio properties can be correspondingly poor. Thus we note that many of the nearer radio-loud galaxies are of early type, although most early-type galaxies are of very low radio luminosity. Frequently, only upper limits to the radio flux are available.

The early-type galaxies, the E's and S0's, clearly differ from the later-type systems in that compact core sources are common in the former. Extended sources comparable to the visible disk are much more common in the later types. When extended sources are found in the early-type galaxies, they are narrow and suggest jet-shaped sources. (Hummel et al. 1984; Condon & Broderick 1988; Condon et al. 1991).

Another strong distinction between ellipticals and later-type systems is found in the comparison of radio with FIR. These two quantities correlate remarkably well for spirals but not for elliptical galaxies and only poorly for S0's (Bregman et al. 1992). This holds over a radio frequency range from at least 151 MHz (Fitt et al. 1988) to 4.85 GHz (Condon et al. 1991) for galaxy types Sa-Im (Sauvage & Thuan 1993). The proposal here is that the infrared measures primarily the reradiation from dust located near sites of active star formation while the radio measures primarily synchrotron radiation from electrons accelerated by supernova remnants resulting from this active massive star formation.