2016A&A...588A.114B

We observed total and polarized radio continuum emission from the spiral galaxy M 101 at λλ6.2cm and 11.1cm with the Effelsberg telescope. The angular resolutions are 2.5 (=5.4kpc) and 4.4 (=9.5kpc), respectively. 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=16kpc (=7.4), where it steepens to an exponential scale length of L≃5kpc, which is about 2.5 times smaller than at R<16kpc. The distribution of the polarized emission has a broad maximum near R=12kpc and beyond R=16kpc also decreases with L≃5kpc. It seems that near R=16kpc 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=16kpc the radial scale length of both fields is about 20kpc, which implies that they decrease to about 0.3µG at R=70kpc, which is the largest optical extent. The equipartition strength of the total field ranges from nearly 10µG at R<2kpc to 4µG at R=22-24kpc. As the random field dominates in M 101 (Bran/Bord≃2.4), wavelength-independent polarization is the main polarization mechanism. We show that energetic events causing HI shells of mean diameter <625pc could partly be responsible for this. At radii <24kpc, the random magnetic field depends on the star formation rate/area, σSFR, with a power-law exponent of b=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 H i filaments.