12.4.2. Polarization
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.
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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.