Magnetic spiral arms in galaxies

Large-scale magnetic fields in spiral galaxies are strongest in spiral-shaped regions, the magnetic arms. It was recently discovered for the galaxy NGC 6946 that magnetic arms can be interlaced with the gaseous arms, rather than coinciding with them. There are indications that the magnetic and gaseous arms may cross in some other galaxies. We suggest that magnetic arms can be located away from regions of higher gas density (gaseous arms) because interstellar turbulence is stronger in the latter. We predict magnetic arms interlaced with gaseous arms in galaxies with weak dynamos, whereas the two spiral structures should overlap in galaxies with strong dynamo action; in an intermediate case, the magnetic and gaseous spiral structures overlap in the inner galaxy and are interlaced in the outer parts (as, possibly, in M51). Another plausible mechanism to produce displaced magnetic and gaseous spiral patterns results from a delay in the dynamo response to the enhancement of turbulence in the gaseous arms. This should lead to the magnetic and gaseous arms crossing at the corotation radius, as possibly observed in the galaxies IC 342 and M83. We also argue that spiral arms only weakly affect the local scaleheight of the galactic gas layer.