Parsec Scale Faraday Rotation in Quasars

A system for making polarization very long baseline interferometry VLBI maps of extragalactic radio sources has been developed over that last decade by the radio astronomy group at Brandeis University. This work represents the first extension of these methods to multiple frequency mapping of the complex polarization of quasars on milliarcsecond scales. By observing at two wavelengths, 3.6 cm and 6 cm, maps of the parsec scale Faraday rotation measures in the quasars 3C 345 and 3C 273 have been produced. While 3C 345 shows little or no Faraday rotation, 3C 273 shows a rotation measure peak of roughly 150 rad-m-2 and a gradient of roughly 90 rad-m-2 in ten parsecs along the jet Ho 100 km-s-1-Mpc-1. The rotation measure maps in combination with the complex polarization images allow the reconstruction of the magnetic field in the parsec scale jets present in these quasars. Both quasars show reconstructed magnetic fields parallel to the jet axis, while the BL Lacertae object OJ 287, observed and mapped as a position angle calibrator, shows a reconstructed magnetic field perpendicular to the parsec scale jet axis. In addition to substantial Faraday rotation, 3C 273 also shows depolarization in the parsec scale jet, with the fractional polarization dropping by 50 between 3.6 cm and 6 cm. These measurements allow a direct estimate of the magnetic field cell sizes in the narrow line region NLR of 3C 273, assuming that the NLR is responsible for the observed depolarization. The cell size is determined to be 0.3 Pc, which is at least four orders of magnitude larger than the upper limits determined from VLA measurements, indicating that the NLR cannot be responsible for the depolarization.