Left panel: CO(2−1) velocity field after excluding those pixels with complex line profiles. Middle panel: best fitting model (see Sect. 4). In these two panels, the contour levels are set in steps of 50 km s^-1 from v(lsrk) = 2800 to 3300 km s^-1. Right panel: residuals (data − model) and the CO(2−1) emission contours as in Fig. 1. The dotted and the dashed lines indicate the major and minor kinematic axes.