Schematic view of M 82’s structure a), as explained in Fig. 6. b) p − v diagram showing the radial velocities of the spiral arms projected along the spectrograph slit. Thin lines: line-of-sight radial velocities of the eastern and western spiral arms; heavy lines: line-of-sight radial velocities of the eastern and western spiral arm sections at the front of the galaxy (i.e. either in front of the dashed line or the dashed-dotted line). Dots on western side: ionized gas [S ii], [N ii] emission lines (Fig. 1c); black dots on eastern side: blue [O ii] ionized gas emission line (Fig. 1e), gray dots on eastern side: blue Hϵ, Hδ stellar absorption lines (Fig. 1f).