Illustration of the procedure used to evaluate the mean Doppler velocity Δv of the narrow component. The velocity spectra summed over the 1369 pixels of the map are shown in black for CO(1−0) (upper panel) and CO(2−1) (lower panel). The quadratic fit over the two control regions is shown in red and its interpolation below the narrow component in blue. The vertical blue lines show the mean Doppler velocities of the narrow component from which the interpolated broad component has been subtracted. They are used as reference for separately evaluating Δv in each pixel.