Radio polarization and magnetic field structure in M 101
  We observed total and polarized radio continuum emission from the spiral
galaxy M 101 at 6.2 cm and 11.1 cm wavelengths with the Effelsberg telescope.
We use these data to study various emission components in M 101 and properties
of the magnetic field. Separation of thermal and non-thermal emission shows
that the thermal emission is closely correlated with the spiral arms, while the
non-thermal emission is more smoothly distributed indicating diffusion of
cosmic ray electrons away from their places of origin. The radial distribution
of both emissions has a break near R=16 kpc, where it steepens to an
exponential scale length of about 5 kpc, which is about 2.5 times smaller than
at R<16 kpc. The distribution of the polarized emission has a broad maximum
near R=12 kpc and beyond R=16 kpc also decreases with about 5 kpc scalelength.
It seems that near R=16 kpc a major change in the structure of M 101 takes
place, which also affects the distributions of the strength of the random and
ordered magnetic field. Beyond R=16 kpc the radial scale length of both fields
is about 20 kpc, which implies that they decrease to about 0.3 \mu G at R=70
kpc, which is the largest optical extent. The equipartition strength of the
total field ranges from nearly 10 \mu G at R<2 kpc to 4 \mu G at R=22-24 kpc.
As the random field dominates in M 101, wavelength-independent polarization is
the main polarization mechanism. We show that energetic events causing HI
shells of mean diameter <625 pc could partly be responsible for this. At radii
<24 kpc, the random magnetic field depends on the star formation rate per area
with a power-law exponent of 0.28+-0.02. The ordered magnetic field is
generally aligned with the spiral arms with pitch angles that are about 8{\deg}
larger than those of HI filaments.
