Continuum-subtracted CO(4−3) and CO(9−8) line of the QSO CTS G18.01 host galaxy (top and bottom panel) and the CO(4−3) line of the MQN01-QC (central panel). The data are reported in yellow. The black solid line shows the best-fit model. The gray shaded area shows the 1σ confidence interval of the best-fit model. The individual Gaussian components are reported in red and blue for the quasar line profile. The green solid line in the central panel shows the line profile extracted from the best-fit rotating disk model as obtained with ^3DBAROLO (see Sect. 4). The fit residuals (data − model) are also reported below each spectra, where the shaded area represents the noise RMS. The bottom velocity scale refers to the quasar systemic redshift as determined by the red component of the quasar CO(4−3) line profile.