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5.3. Rotation measure images 83
correction for the Galactic contribution has been applied.
All of the RM maps show two-dimensional patterns, RM bands, across the lobes with
characteristic widths ranging from 3 to 12 kpc. Multiple bands parallel to each other are observed
in the western lobe of 0206+35, the eastern lobe of 3C 353 and the southern lobe of M 84.
In all cases, the iso-RM contours are straight and perpendicular to the major axes of the lobes
to a very good approximation: the very straight and well-defined bands in the eastern lobes of
both 0206+35 (Fig. 5.2a) and 3C 353 (Fig. 5.2d) are particularly striking. The entire area of M 84
appears to be covered by a banded structure, while in the central parts of 0206+35 and 3C 270 and
the western lobe of 3C 353, regions of isotropic and random RM fluctuations are also present.
I also derived profiles of〈RM〉 along the radio axis of each source, averaging over boxes
a few beamwidths long (parallel to the axes), but extended perpendicular to them to cover the
entire width of the source. The boxes are all large enough to contain many independent points.
The profiles are shown in Fig. 5.5. For each radio galaxy, I also plot an estimate of the Galactic
contribution to the RM derived from a weighted mean of the integrated RM’s for non-cluster radio
sources within a surrounding area of 10 deg 2 (Simard-Normandin et al. 1981). In all cases, both
positive and negative fluctuations with respect to the Galactic value are present.
In 0206+35 (Fig. 5.2a), the largest-amplitude bands are in the outer parts of the lobes, with
a possible low-level band just to the NW of the core. The most prominent band (with the most
negative RM values) is in the eastern (receding) lobe, about 15 kpc from the core (Fig. 5.5a). Its
amplitude with respect to the Galactic value is about 40 rad m −2 . This band must be associated with
a strong ordered magnetic field component along the line of sight. If corrected for the Galactic
contribution, the two adjacent bands in the eastern lobe would have RM with opposite signs and
the field component along the line of sight must therefore reverse.
M 84 (Fig. 5.2b) displays an ordered RM pattern across the whole source, with two wide
bands of opposite sign having the highest absolute RM values. There is also an abrupt change of
sign across the radio core (see also Laing & Bridle 1987). The negative band in the northern lobe
(associated with the approaching jet) has a larger amplitude with respect to the Galactic value than
the corresponding (positive) feature in the southern lobe (Fig. 5.5c).
3C 270 (Fig. 5.2c) shows two large bands: one on the front end of the eastern lobe, the other
in the middle of the western lobe. The bands have opposite signs and contain the extreme positive
and negative values of the observed RM. The peak positive value is within the eastern band at the
extreme end of the lobe (Fig. 5.5e).
The RM structure of 3C 353 (Fig. 5.2d) is highly asymmetric. The eastern lobe shows a
strong pattern, made up of four bands, with very straight iso-RM contours which are almost
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