Channel maps of the HCO+ (4−3) emission (green contours) in velocity intervals of 1 km s⁻¹, covering the total velocity interval from −18.6 to −6.6 km s⁻¹ superimposed on the 870 µm continuum emission (grey tonalities). The velocity interval is indicated in the upper left corner of each image. The contour levels are 0.65 (~6 rms), 1.3, 2.5, 3.5, 5, 7, and 9 K km s⁻¹. The bottom right panel shows the emission of the line integrated into the total velocity range. The contour levels are 0.23 (~7 rms), 0.55, 1.2, 1.8, 5, and 7.2 K km s⁻¹.