First panel: PAWS first moment (velocity) from the CO(1–0) observations by Schinnerer et al. (2013) and upper right panel: analytical velocity field designed to approximate the observations but with purely circular rotation and a constant position angle and inclination. Lower panels: (left) velocity gradient and (right) due to rotation. Contours are at −20, −10, 0, 10, and 20 m s⁻¹ pc⁻¹ for and at −10, −25, and −40 (yellow) m s⁻¹ pc⁻¹ for . The color bars show (top) the velocity and (bottom) the velocity gradient for . For (lower left panel), the straight lines are the 0 m s⁻¹ pc⁻¹ contour which defines the regions discussed in Sect. 3. All velocity gradients are measured in the sky plane (RA–Dec). A more complete explanation of this type of figure can be found in Sect. 5.1 of B18.