Radio polarization and magnetic field structure in M101

Abstract

We observed total and polarized radio continuum emission from the spiral galaxy M 101 at λλ 6.2 cm and 11.1 cm with the Effelsberg telescope. The angular resolutions are 2. 5 (=5.4kpc) and 4. 4 (=9.5 kpc), 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 = 16 kpc (=7. 4), where it steepens to an exponential scale length of L ≃ 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 L ≃ 5 kpc. 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 0.3μG at R = 70 kpc, 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 <625 pc could partly be responsible for this. At radii <24 kpc, the random magnetic field depends on the star formation rate/area, Ssfr, with a powerlaw 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 Hi filaments.