Volume 588, April 2016
|Number of page(s)||15|
|Published online||28 March 2016|
1 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
2 Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland
3 National Astronomical Observatories, Chinese Academy of Sciences, A20 Datun Road, 100012 Beijing, PR China
Received: 7 September 2015
Accepted: 2 January 2016
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 (=5.4 kpc) 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 0.3 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 (Bran/Bord ≃ 2.4), wavelength-independent polarization is the main polarization mechanism. We show that energetic events causing H i 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, 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.
Key words: galaxies: individual: M 101 / galaxies: magnetic fields / galaxies: star formation / radio continuum: galaxies / polarization / radiation mechanisms: non-thermal
FITS files of the images are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (18.104.22.168) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/588/A114
© ESO, 2016
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