AN OPTICAL POLARIZATION SURVEY OF THE PALOMAR-GREEN BRIGHT QUASAR SAMPLE

A broad-band optical polarization survey of all 114 QSOs from the Palomar-Green Bright Quasar Survey reveals a distribution which is peaked near a null value, has a mean <P> = 0.5%, and extends to a maximum value of only P = 2.5%. There is very little evidence for polarimetric variability among objects measured on more than one occasion. The total absence of bona fide blazars among these optically selected quasars is a remarkable result, yet consistent with the weakness of nonthermal components in their overall energy distributions, as indicated by the general lack of high-luminosity, core-dominated radio structures. These characteristics are very similar to those found for Seyfert 1 galaxies. However, among the QSOs, evidence is not as clear that scattering by dust is the dominant polarizing agent. The well-established alignment between polarization position angle and axis of radio emission among radio-loud quasars extends to the less-luminous PG QSOs which exhibit linear radio structures, lending some support to recent unified models of active galactic nuclei. The lack of statistical correlations between polarization and dust indicators for the PG sample as a whole may be due to a combination of polarization mechanisms or to a more diverse distribution of dust in the QSOs than in the Seyfert galaxies.