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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 μG at R=70 kpc, which is the largest optical extent. The equipartition strength of the total field ranges from nearly 10 μG at R<2 kpc to 4 μG 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° larger than those of HI filaments.