RM contours overlaid on the λ6 cm polarisation image. The contour levels are 0 (red), 10 (magenta), 20 (orange), and 30 rad m^-2 (red+magenta) for positive RMs and  − 10 (blue),  − 20 (cyan),  − 30 (green+yellow), and  − 40 rad m^-2 (blue+cyan) for negative RMs. The filled (open) circles represent positive (negative) RMs of extragalactic sources taken from Taylor et al. (2009). The dashed line marks Galactic latitude b = 9 and b = 11.