Magnetic arms in NGC 6946 generated by a turbulent dynamo

Dynamo models for the structure of the regular magnetic field in the galaxy NGC 6946 are confronted with recent observations of polarized radio emission at λ3.5 cm. The observed behaviour of the concentration of regular fields between the gaseous spiral arms (“magnetic spiral arms”) is achieved via a non-axisymmetric azimuthal distribution of only the correlation time τcorr of interstellar turbulence, determined by the typical size of interstellar clouds or the lifetime of supernova remnants or superbubbles. There is no need for the turbulent gas velocity of gas clouds to vary between arm and interarm regions. Furthermore, a satisfactory agreement between model and observations of the radial distribution of the regular field strength requires that the galaxy rotates differentially at radii beyond a few 100 pc, as indicated by observations of the molecular gas. Our model also explains the strong concentration of regular fields within the inner 2 kpc as well as the observed arm/interarm variation of the field pitch angle.