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Chapter 5
Ordered magnetic fields around radio
galaxies: evidence for interaction with
the environment †
M
ost of the RM images of radio galaxies published so far show patchy structures with no
clear preferred direction, consistent with isotropic foreground fluctuations over a range
of linear scales ranging from tens of kpc to< ∼ 100 pc (e.g. 3C 449, Chapter 4; see also Govoni et
al. 2006; Guidetti et al. 2008; Laing et al. 2008; Kuchar & Enßlin 2009; Bonafede et al. 2010),
Numerical modelling has demonstrated that this type of complex RM structure can be accurately
reproduced if the magnetic field is randomly variable with fluctuations on a wide range of spatial
scales, and is spread throughout the whole group or cluster environment (Chapter 4; see also
Murgia et al. 2004; Govoni et al. 2006; Guidetti et al. 2008; Laing et al. 2008; Bonafede et al.
2010). These authors used forward modelling, together with estimators of the spatial statistics of
the RM distributions (structure and autocorrelation functions or a multi-scale statistic) to estimate
the field strength, its relation to the gas density and its power spectrum. The technique of Bayesian
maximum likelihood has also been used for this purpose (Enßlin & Vogt 2005; Kuchar & Enßlin
2009).
In order to derive the three-dimensional magnetic field power spectrum, all of these authors
had to assume statistical isotropy for the field, since only the component of the magnetic field
along the line-of-sight contributes to the observed RM. This assumption is consistent with the
absence of a preferred direction in most of the RM images.
In contrast, the present Chapter reports on anisotropic RM structures, observed in lobed radio
galaxies located in different environments, ranging from a small group to one of the richest clusters
† Guidetti et al. 2011, MNRAS, in press
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