Ordered magnetic fields around radio galaxies: evidence for interaction
  with the environment
  We present detailed imaging of Faraday rotation and depolarization for the
radio galaxies 0206+35, 3C 270, 3C 353 and M 84, based on Very Large Array
observations at multiple frequencies in the range 1365 to 8440 MHz. This work
suggests a more complex picture of the magneto-ionic environments of radio
galaxies than was apparent from earlier work. All of the sources show
spectacular banded rotation measure (RM) structures with contours of constant
RM perpendicular to the major axes of their radio lobes. We give a
comprehensive description of the banded RM phenomenon and present an initial
attempt to interpret it as a consequence of interactions between the sources
and their surroundings. We show that the material responsible for the Faraday
rotation is in front of the radio emission and that the bands are likely to be
caused by magnetized plasma which has been compressed by the expanding radio
lobes. A two-dimensional magnetic structure in which the field lines are a
family of ellipses draped around the leading edge of the lobe can produce RM
bands in the correct orientation for any source orientation. We also report the
first detections of rims of high depolarization at the edges of the inner radio
lobes of M 84 and 3C 270. These are spatially coincident with shells of
enhanced X-ray surface brightness, in which both the field strength and the
thermal gas density are likely to be increased by compression.
