Polarization of radio cores as a test of unified schemes

Abstract

In unified schemes for extragalactic radio sources, quasars and BL Lac objects are intrinsically similar to Fanaroff--Riley class II and I radio galaxies respectively, but appear to be different because they are viewed at different angles to the line of sight. Quasars and BL Lac objects are observed at small viewing angles, while radio galaxies lie close to the plane of the sky. In such schemes, a quasar or BL Lac nucleus is hidden from our view by a putative torus when the source is inclined at a large angle to the line of sight, and it then appears as a radio galaxy. Such a scenario should also affect the observed polarization properties of cores, with the cores in radio galaxies and weak-cored quasars being less polarized than those of core-dominated quasars and BL Lacs because of Faraday depolarization by the magnetoionic medium in the obscuring torus or disc. In this paper, we report that the core polarization of radio galaxies and weak-cored quasars is indeed much smaller than for core-dominated quasars and BL Lacs. We suggest that this is a result of the depolarization caused by the obscuring torus or disc, providing further evidence in favour of unified schemes.