Optical polarization of a complete sample of radio sources

Polarimetry of 50 strong radio sources is presented and combined with VLBI observations and optical spectroscopy in a study of the radio and optical properties of a complete 5 GHz sample. The sample is almost equally divided into radio galaxies and quasars. High polarization (p > 3%) is strongly correlated with the fraction of the total 5 GHz flux density found in a milliarcsecond core (F_c). The fraction of sources with high polarization rises from 13% for sources with F_c < 0.1 to 45% for sources with F_c > 0.1. Few radio galaxies contain optical cores with high polarization, but the detection rate of such cores is consistent with the hypothesis that all radio sources contain optically polarized cores, with strength proportional to the core radio flux density Two-thirds of the known superluminal sources in the sample have high polarization. High optical polarization optical power-law fraction, line-to-continuum ratio, emission lines of small equivalent width, and large amplitude flux variability are all strongly correlated with the fraction of the 5 GHz radio flux density that is unresolved on VLBI scales. The distributions of these optical properties are well matched by a model where the radio and optical radiation have the same beaming geometry, and with Doppler boosting of a factor of 10-100 in the optical. We confirm in this complete sample a striking alignment of the position angle of the VLBI structure axis and the position angle of optical polarization in highly polarized sources. We also find that the highly polarized sources have a large amount of misalignment between the VLBI structure axis and the axis of large-scale radio structure. The properties of weak-lined objects are consistent with the hypothesis that such objects are of two kinds: sources with high isotropic radio and line luminosity and large γ, viewed at an angle to the line of sight much less than 1/γ; and sources of low isotropic radio and line luminosity and moderate γ, viewed at ∼ 1/γ to the line of sight. Various properties of core-dominated sources require a beaming model with a range of Lorentz factors.