Left panel: CO(5 − 4) line image integrating the emisson from −800 to 800 km s⁻¹ (see Sect. 2.2), imaged with the Briggs weighting scheme. Contours are drawn at −3 (dashed), 3, 6, 10, and 14σ (solid; σ = 0.032 Jy beam⁻¹ km s⁻¹). The ellipse in the lower right corner denotes the beam size (0.54” × 0.43”). The FoV is about 3″× 3″ (~ 25 ×25 kpc; 1" corresponds to 8.5 kpc at the redshift of the source). The detected CO(5 − 4) emission is confined to the central ~4 × 4 kpc. Right panel: continuum-subtracted ALMA spectrum extracted around the CO(5 − 4) transition, rebinned at 20 km s⁻¹ per channel. The red filled histogram shows the spectrum extracted from a polygonal aperture encompassing the 3σ contours of the line emission shown in the left panel (roughly corresponding to the 1″ X-shooter slit width; Perna et al. 2015). The dashed lines at −350 and 350 km s⁻¹ are used to define the blue and red tails of the CO emission (see Sect. 6). The purple histogram reproduces the CO(3 − 2) emission from PdBI, taken from Brusa et al. (2015b). The blue line shows the fit with a single Gaussian with FWHM = 550 km s⁻¹.