Left panel: observed velocity gradient. The grayscale image is a zoom into the CO(7–6) map shown in Fig. 1. The blue and red contours correspond to the CO(7–6) line emission integrated in the ranges [ − 135, − 15] and [ − 15,105] km s^-1, respectively. The contours start at 3-σ and are spaced by σ = 104 mJy beam^-1 km s^-1. 2nd to 4th panels: Simulated velocity gradients with two lensed circular Gaussian sources of FWHM ρ = 120 mas, separated by 110 mas and with a position angle of 108, 18 and 63 deg, east from north, respectively (see Sect. 2.3).