Alignment of the optical polarization with the radio structure of QSOs

Using the results of the recently completed linear polarization survey of bright QSOs, we have compared the position angle of linear polarization with the orientation of associated double radio sources. Of the 24 objects for which both position angles are known to better than 15/sup 0/, 20 have the two angles agreeing to within 30/sup 0/. The probability that such alignment occurs by chance is less than 10/sup -5/. On physical grounds, the common orientation of the optical linear polarization and the extended radio lobes indicates a long-term memory of direction in these QSOs, similar to that found in nearby radio galaxies by VLBI observations. The reason for the alignment of the rather small polarization, typically 1%, is unknown. We discuss several possible polarizing mechanisms, including synchrotron radiation near the central source and scattering within the surrounding host galaxy. Measurements of the emission-line polarizations and variability studies of the continuum polarization should be capable of distinguishing between the various sources of polarization and thus may indicate the nature of the observed alignment.