13.1.2. Radio Source Properties
The radio-frequency spectra and polarization properties of radio galaxies and quasars are characteristic of synchrotron radiation from relativistic electrons having a power law distribution of electron energies with a Lorentz factor, ~ 1000 and a magnetic field strength B ~ 10-5 gauss. The radio emission can be conveniently divided into two categories: the extended structure, which is transparent, and the compact structure, where the density of relativistic electrons is so great that the source becomes opaque to its own radiation. There is no simple relation between the structure or dimensions of the radio-emitting region and the dimensions of the associated optical galaxy or quasar, although there are clear statistical differences. Most compact sources are identified with QSOs or with active galactic nuclei. However, less powerful compact sources are also found in normal-looking elliptical galaxies as well (see Ekers 1978, 1981 for a more complete discussion). The extended sources are typically associated with galaxies, but many are quasars with no visible optical extent. Most extended sources, particularly quasars, when examined with sufficient sensitivity and resolution, are found to contain a compact central radio component. The central components are particularly prominent in quasars (e.g., Owen and Puschell 1984). On the other hand, most compact sources, when examined with high sensitivity and dynamic range, exhibit weak extended radio structure. Because the compact sources are affected by self-absorption, their spectra are flat (Section 13.1.3). They are therefore most easily detected by radio surveys made at short wavelengths, whereas the steep-spectrum extended sources with their transparent spectra are characteristic of long-wavelength surveys. The terms extended (or lobe-dominated) source and compact (or core-dominated)source are often used to describe sources where the extended or compact structure, respectively, is most pronounced.
In the less powerful radio galaxies, the radio emission is often confined to the region of optical emission, or about 10 kpc, but in the more powerful radio galaxies, the radio emission comes from two well-separated regions hundreds of kiloparsecs across. In the giant radio galaxies, radio source dimensions larger than 3 Mpc have been observed. The compact features have dimensions typically ranging from 1 to 100 pc, although in a few nearby galaxies radio nuclei as small as 0.01 pc have been observed.