Schematic view of M 82’s structure and radial velocities. a) M 82 seen from the top (north), showing the bar, the stellar disk (gray background) and the trailing spiral arms (from Mayya et al. 2005). The spiral arms may have a width as schematically indicated by the horizontal bars. The arrows indicate the rotation. The bar, hence also the spiral arms, may be viewed under the angle ω ≈ 22° so that the dashed-dotted line is the plane of the sky. b) Black dots: radial velocities of the near-IR stellar Ca ii absorption lines (Fig. 1b) tracing the stellar disk; open circles: radial velocities of the visible ionized gas [N ii], [S ii] lines (Fig. 1c), with the velocity reversal at  ~70′′ west omitted. The dashed lines 1, 2, 3, for example, indicate approximately the region where the radial velocities of the Ca ii lines are measured.