Alma Mater Studiorum Universit`a degli Studi di Bologna ... - Inaf

88 5. Ordered magnetic fields around radio galaxies
0206+35
Figure 5.4: Plots of E-vector position angleΨagainstλ 2 at representative points of the 1.2 arcsec
RM map of 0206+35. Fits to the relationΨ(λ)=Ψ 0 + RMλ 2 are shown. The values of RM are
given in the individual panels.
In order to constrain RM structure on spatial scales below the beamwidth, I estimated the
depolarization as described in Section 5.2. The fitted k values are listed in Table 5.5. I stress that
these values refer only to areas with isotropic fluctuations, and cannot usefully be compared with
the integrated depolarizations quoted in in Fig. 5.2.
For M 84, using the Burn law k analysis and assuming that variation of Faraday rotation
across the 1.65-arcsec beam causes the residual depolarization, I find thatΛ min
< ∼ 0.1 kpc for any
reasonable RM power spectrum.
5.6 Rotation-measure bands from compression
It is clear from the fact that the observed RM bands are perpendicular to the lobe axes that they
must be associated with an interaction between an expanding radio source and the gas immediately
surrounding it. One inevitable mechanism is enhancement of field and density by the shock or
compression wave surrounding the source. 1
The implication of the presence of cavities in the
X-ray gas distribution coincident with the radio lobes is that the sources are interacting strongly
1 An alternative mechanism is the generation of non-linear surface waves (Bicknell, Cameron & Gingold 1990). It
is unlikely that this can produce large-scale bands, for the reasons given in Section 5.8.5.
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