Gaussian decomposition for the CO (6–5) line using up to 3 components. Panel A: single Gaussian fit; panel B: two-components fit; panel C: two-components fit (channels affected by self absorption are masked); panel D: three-components fit (channels affected by self absorption are masked). The spectra, fits and residuals were multiplied by the factor shown in each panel. The Gaussian fits are shown in blue, green and yellow, ordered by their line width; the sum of all components is shown in red. The grey lines indicate the baseline and the dashed horizontal lines placed in the residuals correspond to the 3-σ level. Self-absorption features larger than 5-σ were masked out from the residuals.