Left: polarized emission (contours) at λ20.1 cm at 51′′ resolution, overlaid on a greyscale map of the surface density of the total neutral gas at 55′′ resolution (Crosthwaite et al. 2001) in sky coordinates. Right: the same quantity plotted in polar coordinates (azimuthal angle measured counterclockwise from the north-eastern major axis in the galaxy plane and ln of the radius in arcminutes); the range of azimuthal angles is plotted twice in the right panel for better visibility of the spiral arms.