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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6.5. Summary 127<br />
responsible for the observed RM in the bands and elsewhere. I have presented an initial attempt to<br />
interpret the banded RM phenomenon as a consequence of interactions between the sources and<br />
their surroun<strong>di</strong>ngs. Synthetic RM images have been produced for the case of compression of a<br />
uniformly-magnetized external me<strong>di</strong>um. This simple model of the source-environment interaction<br />
pre<strong>di</strong>cts RM bands of approximately the right amplitude, but only under special initial con<strong>di</strong>tions.<br />
A much better description of the data is provided by a two-<strong>di</strong>mensional magnetic structure in<br />
which the field lines are a family of ellipses draped around the lea<strong>di</strong>ng edge of the lobe. This<br />
draped-field model can produce RM bands in the correct orientation for any source inclination.<br />
Moreover, it might explain the high RM amplitude and low depolarization observed within the<br />
bands. The two-<strong>di</strong>mensional draped-field model may also account for the low rate of detection of<br />
RM bands in published observations, since anisotropic RM structures will not be observed unless<br />
the line-of-sight intercepts the wrapped field lines. Furthermore, superposed RM fluctuations with<br />
significant power on scales comparable with the bands will make them <strong>di</strong>fficult to recognise.<br />
This work has demonstrated unequivocally for the first time that the interaction between ra<strong>di</strong>o<br />
galaxies and their imme<strong>di</strong>ate environments affects the magnetisation of the surroun<strong>di</strong>ng plasma.<br />
Finally, I have reported rims of high depolarization at the edges of the inner ra<strong>di</strong>o lobes of<br />
M 84 and 3C 270. Such highly depolarized structures had not been observed in earlier, less detailed<br />
work. The magnetic fields responsible for such depolarization must be tangled on small scales,<br />
since they produce depolarization without any obvious effects on the large-scale Faraday rotation<br />
pattern. The rims are spatially coincident with shells of enhanced X-ray surface brightness, in<br />
which both the field strength and the thermal gas density are likely to be increased by compression.<br />
6.5.3 The ra<strong>di</strong>o galaxies 0755+37 and M 87<br />
I have presented two-<strong>di</strong>mensional analyses of the RM and depolarization across the ra<strong>di</strong>o galaxy<br />
0755+37, in a very poor group, and the inner 5 kpc of the ra<strong>di</strong>o structure of M 87, located at the<br />
centre of the cool-core Virgo cluster. 0755+37 shows the clearest known example of jet-related<br />
RM structure. The amplitudes of the RM fluctuations across 0755+37 and M 87 are typical of their<br />
environments: in 0755+37 they are smaller than those detected in sources in groups of galaxies and<br />
much more than those in M 87, which are typical of a cool core cluster. The RM structure functions<br />
have been evaluated for both sources over sub-regions where the RM fluctuation amplitudes are<br />
approximately constant. The structure-function analysis has shown that the best-fitting power<br />
spectrum of the RM fluctuations (and thus the magnetic field) is a power law with a cut-off at high<br />
spatial frequencies. The best-fitting power spectra are <strong>di</strong>fferent for the two lobes of 0755+37: the<br />
slopes are q≃2.5 in the eastern lobe and q≃2.2 in the west. The minimum field scale derived<br />
from depolarization measurements is 0.1 kpc and the maximum field fluctuation scale is≈50 kpc.<br />
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