Velocity structure of CO (left panel) and HCO+ (right panel) line emission (> 10 mJy beam⁻¹) around the D1 region. The two black crosses are the OH emission regions OH1 and OH2, as shown in Fig. 1. The green and blue contours stand for the CO and HCO+ emissions, respectively. The contour levels are present in the caption of Fig. 9. The blue spots indicate where the extracted spectrum shows separated double peaks, roughly distributed in six regions (DP1–DP6).