Faraday rotation in the CSS QSOs 3C 43 and 3C 454

We present observations of the CSS QSOs 3C 43 and 3C 454 made with the VLA, MERLIN and VLBA at 2, 6 and 18 cm to examine the Faraday rotation in these highly bent sources. 3C 43 is weakly polarized at 18 cm at mas resolution but has a large rotation measure near the bend in the jet. This result is interpreted as indicating jet–ISM interaction. 3C 454 is relatively strongly polarized and has high rotation measure along the jet visible in the high resolution 18 cm images. There is no brightening of the jet near the bend to the west indicating that it is not caused by a jet–cloud collision and may simply be a hydrodynamic instability magnified by a small angle to the line of sight. However, there are clear differences in the sign and magnitude of the Faraday rotation measured at long and short wavelengths. This is interpreted as being due to thermal plasma mixed with the jet. Likely detection of circular polarization at 18 cm supports this interpretation.