Magnetism in the spiral galaxy NGC 6946: magnetic arms, depolarization rings, dynamo modes and helical fields

Context. The spiral galaxy NGC 6946 hosts magnetic spiral arms, highly aligned magnetic fields between the gas/optical arms. Aims. The origin of the magnetic phenomena and their relation to the interstellar gas are investigated. Methods. NGC 6946 was observed in total intensity and linear polarization in five radio bands between 3 cm and 21 cm. Maps of spectral index, Faraday rotation and depolarization were derived. Results. At the inner edge of the inner gas spiral arm the ordered magnetic field is only mildly compressed and turns smoothly, to become aligned along the gas arm. Hence the field is not shocked and is probably connected to the warm, diffuse gas. At larger radii, two bright magnetic arms between the optical arms are visible in polarized intensity. The field in the northern magnetic arm is almost totally aligned. Faraday rotation measures (RM) in these arms are consistent with the superposition of two low azimuthal dynamo modes. Three more magnetic arms are discovered in the outer galaxy, located between H I arms. The RM structure function confirms large-scale coherent fields. The observed anti-correlation between the field’s pitch angles and the RM values is a possible signature of helical fields. – Due to strong Faraday depolarization the galaxy is not transparent to polarized waves at λ18 cm and λ20 cm. The large-scale asymmetry in depolarization with respect to the major axis may be another indication of large-scale helical fields. Three depolarization rings of almost zero polarization seen at λ20 cm are probably generated by differential Faraday rotation in H II complexes in NGC 6946 of 300–500 pc size. – In the gas/optical spiral arms, the total (mostly turbulent) magnetic field is amplified to ≃ 20 μG. Its energy density is ≃10 times larger than that of the ionized gas and is similar to that of the turbulent gas motions in the inner galaxy. The magnetic energy exceeds that of the turbulent energy in the outer galaxy. All energy densities in NGC 6946 are about one order of magnitude larger than those in the Milky Way. Conclusions. Density waves in the inner gaseous spiral arms mildly compress the field. Dynamo action probably generates the magnetic spiral arms. The magnetic field is dynamically important, interacts with the gas flow and possibly determines the properties of the gas spiral arms.