Moment 0 map of the CO(1–0) emission at the resolution of the HCO⁺ observations. The red contours are those of the HCO⁺ emission. The solid line and dashed circles correspond to the field of view and the FWHM of the HCO⁺ primary beam, respectively. The numbers are the clump labels. Subpanel: Position of the HCO⁺ field of view with respect to the Hi@ shell (black contours) and the radio continuum (grey contours). A larger spatial overview is provided in Fig. 1 of Oosterloo & Morganti (2005), where the region is labelled ‘outer filament’.