A View through Faraday’s Fog: Parsec‐Scale Rotation Measures in Active Galactic Nuclei

Rotation measure observations of nine quasars, four BL Lacertae objects, and three radio galaxies are presented. The rest-frame rotation measures in the cores of the quasars and the jets of the radio galaxy M87 are several thousand rad m -2 . The BL Lac objects and the jets of the quasars have rest-frame rotation measures of a few hundred rad m -2 . A core rotation measure of 500 rad m -2 in the rest frame is suggested as the dividing line between quasars and BL Lac objects. The substantial rotation measures of the BL Lac objects and quasars cast doubt on the previous polarization position angle investigations of these objects at frequencies of 15 GHz or less. BL Lac itself has a rotation measure that varies in time, similar to the behavior observed for the quasars 3C 273 and 3C 279. A simple model with magnetic fields of 40 μG or less can account for the observed rotation measures.