Left panel: radial intensity profile as a function of anticlockwise angle through the centre of the channel at υ = 0 km s⁻¹, starting at a position angle of 150^° (anticlockwise from north). Flux units are in Jy beam⁻¹. The spiral arm is visible as the radially expanding blue filament. White dotted lines have been overplotted to indicate the trend an Archimedean spiral would follow with a spiral arm separation of ~1″ (or 135 AU at a distance of 135 pc). The spiral signal seems not to propagate into the inner regions (<1.5″), and dilutes around 5″. At around 10″ a relatively sharp edge is seen in the signal, in approximately every radial direction. This is indicated by the red dashed line. Right panel: example of a radial intensity profile (at a position angle of 60^°) to better show the intensity drop at ~10″.