A View through Faraday ’ s Fog 2 : Parsec Scale Rotation Measures in 40 AGN

Results from a survey of the parsec scale Faraday rotation measure properties for 40 quasars, radio galaxies and BL Lac objects are presented. Core rotation measures for quasars vary from approximately 500 to several thousand rad m . Quasar jets have rotation measures which are typically 500 rad m or less. The cores and jets of the BL Lac objects have rotation measures similar to those found in quasar jets. The jets of radio galaxies exhibit a range of rotation measures from a few hundred rad m to almost 10,000 rad m for the jet of M87. Radio galaxy cores are generally depolarized, and only one of four radio galaxies (3C120) has a detectable rotation measure in the core. Several potential identities for the foreground Faraday screen are considered and we believe the most promising candidate for all the AGN types considered is a screen in close proximity to the jet. This constrains the path length to approximately 10 parsecs, and magnetic field strengths of approximately 1 μGauss can account for the observed rotation measures. For 27 out of 34 quasars and BL Lacs their optically thick cores have good agreement to a λ law. This requires the different τ = 1 surfaces to have the same intrinsic polarization angle independent of frequency and distance from the black hole. Subject headings: galaxies: active – galaxies: ISM – galaxies: jets – galaxies: nuclei – radio continuum: galaxies National Radio Astronomy Observatory, P.O. Box 0, Socorro, NM 87801 Department of Astronomy, New Mexico State University, MSC 4500 P.O. Box 30001, Las Cruces, NM 88003-8001 Present address: U. S. Naval Observatory Flagstaff Station, P.O. Box 1149 Flagstaff, AZ 86002-1149