The optical polarization properties of quasars

Optical polarimetry is presented for 163 radio-selected quasars, including a complete sample of 90 sources stronger than 2 Jy at 5 GHz. Including new and published data, the polarimetry is over 90% complete. Synchrotron components are detected in over 30% of these flat spectrum quasars, with an optical polarization threshold of p > 3%. The fraction of polarized quasars is a strong function of the compactness of the radio emission, as measured on VLBI scales. Including the duty cycle correction, essentially every quasar with S_core > S_ext at radio wavelengths has a prominent blazar component at optical wavelengths. Virtually all of the radio sources with weak emission lines (or BL Lac objects) are highly polarized. Optical and radio polarization are not correlated, and there is no difference in the α_ro distributions of high-and low-polarization quasars, or sources with strong and weak emission lines. Redshift information is incomplete, but BL Lac objects appear to have lower redshifts than strong-lined quasars. The distribution of V/V_max for radio BL Lac objects indicates much stronger evolution than is found for X-ray-selected BL Lac objects. Overall, there is a strong statistical link between compact radio structure, apparent superluminal motion, strong optical polarization, and weak emission lines. This can be understood if both the optical and the compact radio emission are relativistically beamed. However, the Lorentz factors of the material emitting the optical and radio radiation must be different.