8.7. Optical Polarization

Polarization measurements for GPS and CSS sources (mostly quasars) compiled from the literature are shown in Table 7. In general the polarizations are low ( 1%). The fact that the GPS/CSS polarizations are so low compared to the polarizations of core-dominated quasars led Wills et al. (1992) to suggest that the GPS and CSS quasars are not as strongly beamed as the core-dominated quasars. These low polarizations are comparable to those found in rest-frame optical observations of extended lobe-dominated radio galaxies (see, e.g., Antonucci 1984; Tadhunter et al. 1992). These observations are consistent with a scattered (in galaxies) or beamed (in quasars) optical component generally being weak or diluted in GPS and CSS sources. However, in marked contrast to this trend is PKS 0116+082, which is a variable and strongly polarized CSS NLR galaxies (Cohen et al. 1997).

Another interesting counterexample is the archetypal compact double source 1934-638. Tadhunter, Shaw, & Morganti (1994) detected optical polarization of ~ 3.5% in 1934-638. The polarization E-vector is oriented within 10° of perpendicular to the axis of the compact double radio source. Tadhunter et al. suggest that the observed polarization is probably scattered nuclear light, though dichroic absorption in a dust band perpendicular to the radio axis and synchrotron emission associated with the radio source are also possible. Polarization observations are needed of other GPS and CSS galaxies, especially those exhibiting the alignment effect (section 8.3).