Nearly all of the radio galaxies and quasars have some degree of linear polarization ranging from integrated values of a few tenths of a percent to several percent, with the greatest value about 20%. At least for the extended sources, the integrated polarization is generally greatest at the shorter wavelengths, and the greatest polarization is found in the low surface brightness objects. In general the plane of polarization rotates at a rate approximately proportional to 2 and it is generally considered that this is due to Faraday rotation. Since the amplitude and sign of rotation appear to depend on galactic coordinates, it is thought that much, but not necessarily all, of the rotation occurs within the Galaxy. The degree of depolarization at longer wavelengths may also depend on galactic coordinates, but this is not clearly established.
Observations have also been made to map the distribution of polarized emission. In some cases the observed polarization reaches a degree of polarization comparable to that expected from a uniform magnetic field, indicating remarkably aligned magnetic fields over large volumes of space. Usually the regions of lowest surface brightness show the greatest polarization. Although there is no simple general relationship between the source geometry and the polarization direction, in many sources the polarization appears to be either parallel or perpendicular to the direction of elongation. In several sources the polarization is radial suggesting a circumferential magnetic field (Fomalont 1973). Figure 12.4 shows the polarization vectors observed for Fornax A at a 6-cm wavelength.
Figure 12.4 Polarization vectors of Fornax A observed at 6 cm superimposed on contours of total brightness temperature. The resolution was 4 minutes of arc beam using the Parkes 210-foot radio-telescope. In the eastern component the polarization is very low at the intensity maximum, while in the other it is much greater. (Taken from Gardner and Whiteoak (1971), Australian J. Phys. 29:899].
Recently, several observers have detected small amounts of circular polarization in a few compact sources.