1999AJ....118.1931V

Multifrequency polarimetric observations made with the Very Long Baseline Array of the quasar 3C 216 reveal the presence of Faraday rotation measures (RMs) in excess of 2000 rad.m^–2 in the source rest frame in the arc of emission located at ∼140 mas from the core. Rotation measures in the range -300 to +300 rad.m^–2 are detected in the inner 5 mas (∼30 pc). While the rotation measure near the core can be explained as being caused by a magnetic field in the narrow-line region, we favor the interpretation for the high RM in the arc that it is caused by a ``local'' Faraday screen produced in a shock where the jet is deflected by the interstellar medium of the host galaxy. Our results indicate that a galactic magnetic field of the order of ∼50 µG on a scale greater than 100 pc must be present in the ambient medium.