THE MAGNETIC-FIELD IN THE HALO OF M82 - POLARIZED RADIO-EMISSION AT LAMBDA-LAMBDA-6.2 AND 3.6 CM

The first detection of linearly polarized radio emission of the starburst galaxy M 82 at lambdalambda6.2 and 3.6 cm is reported. A rotation measure analysis allows the delineation of the magnetic field structure in the synchrotron halo. Also, our polarization images at these 2 wavelengths are sensitive to magnetoionic material at the lowest density levels. The overall appearance of the magnetic field orientation strongly suggests a connection of the field lines with the streaming motions of the synchrotron emitting plasma, i.e. the magnetic field is controlled by the motion of the relativistic plasma and is convected into the M 82 halo. In particular, the northern halo exhibits a poloidal field structure, in agreement with earlier results. At lambda3.6 cm linear polarization of up to 35% is found, with the fractional polarization increasing with z-distance. Analysis of the rotation measure distribution between lambdalambda6.2 and 3.6 cm hints at a source geometry in which the observed polarized (i.e. not depolarized) radio emission originates mainly in a layer surrounding the outflowing Halpha gas. We find the center of M 82 to be completely depolarized, as expected, due to differential Faraday rotation within the dense magneto-ionic medium. The discussion about the origin of the large-scale magnetic field in M 82 leads to the conclusion that the origin of the field structure cannot be attributed to the action of an alphaomega-dynamo in the galactic disk, but is more likely due to plasma streaming caused by the violent star-forming activity in the center of the galaxy.