SiO (colour scale), CS (white contours) and H₂CO (black contours) position-velocity diagram for a cut along the grey line shown in Figs. 2 and 5. The units of the colour scale are Jy/beam, and the contours of the CS and H₂CO emission correspond to 10% to 90% of the peak value, in steps of 20%. The SiO emission in particular shows both high and low velocity components, while the CS traces an arc with both high and low velocity gas. These patterns are best seen in the blue shifted material.