Are rotation curves in NGC 6946 and the Milky Way magnetically supported?

The inner disk rotation of NGC 6946 and the Milky Way is dominated by gravity but magnetism is not negligible at radii where the rotation curve becomes flat, and indeed could become dominant at very large radii. Values of the order of I mu G, or even less, produce a centripetal force when the absolute value of the slope of the curve [B,,R] (azimuthal field strength versus radius) is less than the slope of a B-phi-profile proportional to R-1. The proportional to R-1-profile is here called the critical profile. From the hypothesis of magnetically driven rotation curves, the following is to be expected: at large radii, a "subcritical" profile (slope flatter than R-1); at still larger radii a B-phi-profile becoming asymptotically critical as the density becomes asymptotically vanishing. Recent observations of magnetic fields in NGC 6946 and the Milky Way are in very good agreement with these predictions.