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74 5. Ordered magnetic fields around radio galaxies
of galaxies. The RM images of radio galaxies presented in this Chapter show clearly anisotropic
“banded” patterns over part or all of their areas. In some sources, these banded patterns coexist
with regions of isotropic random variations. The magnetic field responsible for these RM patterns
must, therefore, have a preferred direction.
One source whose RM structure is dominated by bands is already known: M 84 (Laing &
Bridle 1987). In addition, there is some evidence for RM bands in sources which also show strong
irregular fluctuations, such as Cygnus A (Carilli & Taylor 2002). It is possible, however, that some
of the claimed bands could be due to imperfect sampling of an isotropic RM distribution with
large-scale power, and I return to this question in Section 5.8.1.
In this Chapter I present new RM images of three sources which show spectacular banded
structures, together with improved data for M 84. The environments of all four sources are
well characterized by modern X-ray observations, and I give the first comprehensive description
of the banded RM phenomenon. I present an initial attempt to interpret the phenomenon as a
consequence of source-environment interactions and to understand the difference between it and
the more usual irregular RM structure.
The RM images reported are derived from new or previously unpublished archive Very Large
Array data for the nearby radio galaxies 0206+35, M 84 (Laing et al. in preparation), 3C 270
(Laing, Guidetti & Bridle in preparation) and 3C 353 (Swain, private communication; see Swain
1996).
5.1 The Sample
High quality radio and X-ray data are available for all of the sources. In this Section I summarize
those of their observational properties which are relevant to this RM study. A list of the sources and
their general parameters is given in Table 5.1, while Table 5.2 shows the X-ray parameters taken
from the literature and equipartition parameters derived from radio observations in this work.
The sources were observed with the VLA at several frequencies, in full polarization mode and
with multiple configurations so that the radio structure is well sampled. The VLA observations,
data reduction and detailed descriptions of the radio structures are given for 0206+35 and M 84
by Laing, Guidetti et al. (in preparation), for 3C 270 by Laing, Guidetti & Bridle (in preparation),
and for 3C 353 by Swain (1996). All of the radio maps show a core, two sided jets and a doublelobed
structure with sharp brightness gradients at the leading edges of both lobes. The synchrotron
minimum pressures are all significantly lower than the thermal pressures of the external medium
(Table 5.2).
All of the sources have been observed in the soft X-ray band by more than one satellite,
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