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118 6. Faraday rotation in two extreme environments
0 20000 40000 60000
12 24 00
12 23 40
DECLINATION (J2000)
12 23 20
12 23 00
12 22 40
12 30 51
12 30 50
12 30 49
12 30 48
12 30 47
RIGHT ASCENSION (J2000)
Figure 6.8: Left: Image of Burn law k for M 87 computed from weighted least-squares fits to the
relation p(λ)= p(0)exp(−kλ 4 ) at the eight frequencies listed in Table 6.3. Right: Profiles of Burn
law k along the jet direction, plotted against distance from the nucleus. The angular resolution is
0.4 arcsec FWHM.
The Burn law k map for M 87 is shown in Fig. 6.8. At 0.4 arcsec resolution the overall k
distribution has a mean of 9820 rad 2 m −4 , corresponding to a depolarization DP 6cm
3cm
= 0.85. The
mean observed k values for the two lobes are listed in Table 6.4. The highest depolarization occurs
in patchy structures in the E (receding) lobe, consistent with the Laing-Garrington effect (Laing
1988; Garrington et al. 1988). These structures appear filamentary and are likely to be caused by
steep (and therefore unresolved) RM gradients. Indeed, they are in the same areas as the highest
absolute values of RM and∇RM (Fig. 6.7).
In addition to low and constant RM, the jet also shows uniformly low k.
The profile of
〈k〉 across the source (Fig. 6.8, right) is qualitatively similar to that observed in 0755+37, but
more extreme: it increases monotonically from W to E across the core, peaks in the E lobe
and then decreases at larger distances. Except in the region of the jet, no detailed correlation
of depolarization with source morphology is apparent. Together with the absence of significant
deviations fromλ 2 rotation, this again requires a Faraday rotating medium mostly external to the
source.
6.3 Two-dimensional analysis: structure functions
I used the RM structure function (Eq. 3.13) to quantify the two-dimensional fluctuations of Faraday
rotation measure on scales larger than the observing beamwidth for 0755+37 and M 87.
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