Polarised flux density contours from the smoothed RM cube of the combined λ18 cm and λ22 cm data overlaid on a Hα image from the WIYN Observatory. The contour levels are at 200, 250, 300, and 400 μJy beam^-1, which corresponds to a 5σ noise level in the individual Stokes U- and Q cubes. In the lower left we show the size of the synthesised beam (43.̋1 × 39.̋6). The red vectors indicate the measured polarised flux density (1″ = 40 μJy beam^-1) and the intrinsic magnetic field direction of the emission. The green cross marks the optical centre of M 82 and the blue ellipse the extent of the central optical disk.