a) Schematic representation of measured ¹²CO(1–0) radial velocities (from Walter et al. 2002, interferometer measurements). Overlaid on the eastern side: velocity reversal observed in stellar motions (Fig. 1f); western side: velocity reversal observed in [S ii], [N ii] emission lines (Fig. 1c). b) ¹²CO (2–1) radial velocities (contours at 0.03, 0.1, 0.5, 1, 2.5, 4, and 10 K). Overlaid are radial velocities of stars: black dots (Fig. 1b) and of stellar clusters: open circles (from Konstantopoulos et al. 2009, and Fig. 4b). The line in a) and b) is the smooth ¹²CO(2–1) radial velocity curve published by Sofue (1998). The IRAM 30-m telescope CO data shown in b) are from Weiß et al. (2009, priv. comm.) [HERA 9-channel receiver, 11′′ beam width].