Faraday rotation variations along radio jets: the magnetic field in galaxy and group halos

Our modelling of FRI radio jets as decelerating, relativistic flows allows us to derive their orientations accurately. We present images of Faraday rotation for two of these these objects (3C 31 and NGC 315) and show that the fluctuations of rotation measure (RM) are larger in the fainter (receding) jets, as expected if the rotation occurs in the hot galaxy/group halos. The gas density is much lower in NGC 315 and the RM fluctuations are only just detectable. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.