Alma Mater Studiorum Universit`a degli Studi di Bologna ... - Inaf
Alma Mater Studiorum Universit`a degli Studi di Bologna ... - Inaf
Alma Mater Studiorum Universit`a degli Studi di Bologna ... - Inaf
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5.8. Discussion 99<br />
5.8 Discussion<br />
5.8.1 Where do bands occur?<br />
The majority of published RM images of ra<strong>di</strong>o galaxies do not show bands or other kind of<br />
anisotropic structure, but are characterized by isotropic and random RM <strong>di</strong>stributions (e.g. Laing<br />
et al. 2008, 3C 449, Chapter 4). On the other hand, I have presented observations of RM bands<br />
in four ra<strong>di</strong>o galaxies embedded in <strong>di</strong>fferent environments and with a range of jet inclinations<br />
with respect to the line-of-sight. These sources are not drawn from a complete sample, so any<br />
quantitative estimate of the incidence of bands is premature, but it is possible to draw some<br />
preliminary conclusions.<br />
The simple two-<strong>di</strong>mensional draped-field model developed in Section 5.7.3 only generates<br />
RM bands when the line-of-sight intercepts the volume containing elliptical field lines, which<br />
happens for a restricted range of rotation around the source axis. At other orientations, the RM<br />
from this field configuration will be small and the observed RM may well be dominated by<br />
material at larger <strong>di</strong>stances which has not been affected by the ra<strong>di</strong>o source. I therefore expect<br />
a minority of sources with this type of field structure to show RM bands and the remainder to have<br />
weaker, and probably isotropic, RM fluctuations. In contrast, the three-<strong>di</strong>mensional draped field<br />
model proposed by Dursi & Pfrommer (2008) pre<strong>di</strong>cts RM bands parallel to the source axis for a<br />
significant range of viewing <strong>di</strong>rections: these have not (yet) been observed.<br />
The prominent RM bands here described occur only in lobed ra<strong>di</strong>o galaxies. In contrast, wellobserved<br />
ra<strong>di</strong>o sources with tails and plumes seem to be free of bands or anisotropic RM structure<br />
(e.g. 3C 31, 3C 449; Laing et al. 2008, Chapter 4). Furthermore, the lobes which show bands are all<br />
quite round and show evidence for interaction with the surroun<strong>di</strong>ng IGM. It is particularly striking<br />
that the bands in 3C 353 occur only in its eastern, rounded, lobe. The implication is that RM bands<br />
occur when a lobe is being actively driven by a ra<strong>di</strong>o jet into a region of high IGM density. Plumes<br />
and tails, on the other hand, are likely to be rising buoyantly in the group or cluster and I do not<br />
expect significant compression, at least at large <strong>di</strong>stances from the nucleus.<br />
5.8.2 RM bands in other sources<br />
The fact that RM bands have so far been observed only in a few ra<strong>di</strong>o sources may be a selection<br />
effect: much RM analysis has been carried out for galaxy clusters, in which most of the sources<br />
are tailed (e.g. Blanton et al. 2003). With a few exceptions like Cyg A (see below, Section 5.8.2),<br />
lobed FR I and FR II sources have not been stu<strong>di</strong>ed in detail.<br />
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