Figure 1 gives an example of observations of M51
at 4 different wavelengths, all smoothed to the same linear resolution of
75" HPBW. The vectors are rotated by 90° but not corrected
for Faraday rotation. The figure illustrates nicely the different
effects of Faraday rotation and depolarization effects depending on
the observing wavelength: the observed vectors at
2.8 cm and
6 cm are mainly
parallel to the optical spiral arms as
expected in spiral galaxies (see below), Faraday rotation is small at
centimeter wavelengths. However, the pattern looks very different at
18/20 cm where Faraday
rotation is expected to be strong.
Further, we see a region in the northeastern part of M51 with complete depolarization.
![]() |
Figure 1. Maps of the E-vectors rotated by
90° of M51 observed at
|
After substraction of the thermal fraction of the emission we
distinguish between beam-dependent and wavelength-dependent
depolarization. The difference in depolarization at different
wavelengths in maps with the same linear resolution should be
purely wavelength dependent where two different wavelength-dependent
depolarization effects are important to consider: the differential
Faraday rotation and Faraday dispersion as despcribed by
Burn (1966) and
Sokoloff et al. (1998).
The latter effect is due to turbulent
magnetic fields within the source and between the source and us,
whereas the Faraday rotation depends on the regular magnetic field
within the emitting source. The differential Faraday rotation has a
strong wavelength dependence as shown e.g. in
Fig. 1 in
Sokoloff et al. (1998)
leading to a complete depolarization at
20 cm already at a
RM
40 rad /
m2, with again
decreasing depolarization for higher RMs. Such an effect has first been
detected in small isolated areas in M51
(Horellou et al. 1992).
At
6 cm the
depolarization is much smaller, increasing smoothly to zero at
RM
400 rad /
m2 (the first zero point is at
RM =
/ (2 .
2)).
Hence, the galaxies may not be transparent in linear polarization at decimeter wavelengths so that we may observe just an upper layer of the whole disk. At centimeter wavelengths we do not expect complete depolarization even in galaxies viewed edge-on, i.e. centimeter wavelengths are best suitable to trace the magnetic field structure.