Left: faraday rotation measures RM between λ6.2 cm and λ11.2 cm (Effelsberg only) at 5′ resolution, averaged in sectors of 10° azimuthal width in the plane of the galaxy (counted counterclockwise from the north-eastern major axis) for the ring 7.́5–12.́5. The error bars are computed from the standard deviations of the polarization angles at the two wavelengths. The parameters of the fitted line are given in Table 5. Right: same for the ring 12.́5–17.́5.