M 81 at high radio frequencies.

The total radio continuum and linearly polarized emission of M81 at 6.3 cm and 2.8 cm have been mapped with the Effelsberg 100-m telescope. The total emission is concentrated around 5 kpc distance from the center, near the inner parts of the spiral arms, and decreases outward with 3.0 kpc scale length. If the cosmic ray electrons originate in the spiral arms, they have to propagate outwards with 150 km/s which may represent the Alfven speed in the outer parts of M81. The degree of polarization of the synchrotron emission at 6.3 cm is 14 + or - 6 percent on average over the galaxy, but reaches 50 percent along the western spiral arm. This is the highest degree of polarization on a scale of several kpc detected in a nearby galaxy so far. Large-scale shock fronts (e.g., density waves) are considered to align the magnetic field almost totally along the spiral arms. Where the magnetic field can be traced it is found to follow the spiral arms.