The galactic magnetic field in the quasar 3C 216

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 are of emission located at similar to 140 mas from the core. Rotation measures in the range - 300 to + 300 rad m(-2) are detected in the inner 5 mas (similar to 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 are 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 similar to 50 mu G on a scale greater than 100 pc must be present in the ambient medium.