Positions observed in HCO+(1–0), HCN(1–0) and ¹³CO(2–1) (and C¹⁸O) are indicated with circles superimposed on the Herschel/PACS 100 μm map (Viaene et al. 2014). Contour levels (0.05, 0.1, 0.2) correspond to A_B extinction computed assuming all the dust is in front of the bulge as discussed in Melchior et al. (2000). Extinction is underestimated by an unknown factor due to the uncertainties in the geometrical configuration of the gas in this area.