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

90 5. Ordered magnetic fields around radio galaxies
IMAGE: K;BURN_3p_4.5_BLANKED.FITS
CNTRS: M84_CHANDRA_NOPTS_AS_1.3_4.5.FITS.3
0 100 200 300 400 500 600
IMAGE: 3C270_KBURN_5.0.FITS
CNTRS: XMM_12_ARCSEC_AS_1.3_5.0.FITS
rad^2/m^4
m
0 100 200 300 400 500 600
rad^2/m^4
rad m
12 54 00
05 52 00
DECLINATION (J2000)
12 53 00
DECLINATION (J2000)
05 50 00
05 48 00
12 52 00
(a)
12 25 09
12 25 06 12 25 03
RIGHT ASCENSION (J2000)
12 25 00
(b)
12 19 40
12 19 30 12 19 20
RIGHT ASCENSION (J2000)
12 19 10
Figure 5.6: Burn law k images of M 84 (a) and 3C 270 (b) overlaid on X-ray contours derived
from Chandra and XMM-Newton data, respectively.
with the thermal gas, displacing rather than mixing with it (see McNamara & Nulsen 2007 for
a review). For the sources here presented, the X-ray observations of M 84 (Finoguenov et al.
2008, Fig.5.1b) and 3C 270 (Croston et al. 2008, Fig.5.1c) show cavities and arcs of enhanced
brightness, corresponding to shells of compressed gas bounded by weak shocks. The strength of
any pre-existing field in the IGM, which will be frozen into the gas, will also be enhanced in the
shells. Therefore a significant enhancement in RM is expected. A more extreme example of this
effect will occur if the expansion of the radio source is highly supersonic, in which case there will
be a strong bow-shock ahead of the lobe, behind which both the density and the field become much
higher. Regardless of the strength of the shock, the field is modified so that only the component in
the plane of the shock is amplified and the post-shock field tends become ordered parallel to the
shock surface.
The evidence so far suggests that shocks around radio sources of both FR classes are generally
weak (e.g. Forman et al. 2005, Wilson, Smith & Young 2006, Nulsen et al. 2005a). There are
only two examples in which highly supersonic expansion has been inferred: the southern lobe of
Centaurus A (M≈8; Kraft et al. 2003) and NGC 3801 (M≈4; Croston et al. 2007). There
is no evidence that the sources here described are significantly over-pressured compared with
the surrounding IGM (indeed, the synchrotron minimum pressure is systematically lower than the
thermal pressure of the IGM; Table 5.2). The sideways expansion of the lobes is therefore unlikely
to be highly supersonic. The shock Mach number estimated for all the sources from ram pressure
balance in the forward direction is also≈1.3. This estimate is consistent with that for M 84 made
by Finoguenov et al. (2006) and also with the lack of detection of strong shocks in the X-ray data
90