2001ApJ...559..703T

We present sensitive, high dynamic range, Very Long Baseline Array (VLBA) polarimetric observations of the cores of three powerful radio galaxies: 3C 166, 3C 236, and 3C 390.3. Significant polarization is detected in one source (3C 166) allowing us to map out the Faraday rotation measure (RM) distribution and projected magnetic field direction. The inner jet of 3C 166 is found to have a rest frame RM of -2300 rad m^–2, similar to those found in quasar cores. No polarized flux was detected from the other two sources, but in both, counterjets are seen. The counterjet in 3C 236 was previously known, but the detection in 3C 390.3 is a new discovery. We suggest that the low fractional polarization in radio galaxy cores is the result of Faraday depolarization by ionized gas associated with the accretion disk. The lower polarization of radio galaxy cores compared to quasars is then naturally explained by unified models as a result of the viewing angle.