Zoom-in view of the spectrum around CO(4−3), integrated in a polygon set by the > 2σ noise level around the QSO. A double Gaussian fit was applied to CO(4−3), including a systemic (blue) and a blueshifted (green) line component. The dashed line shows the combination of the two Gaussian functions. The continuum was fitted with a linear function and subtracted as detailed in Sect. 3.2.