Upper panel: CO excitation ladder of XID2028 (stars) compared with average values obtained for various classes of sources (ULIRGs, SMGs, BzKs, and the Milky Way, as labelled; taken from Papadopoulos et al. 2012; Bothwell et al. 2013; Daddi et al. 2015; Fixsen et al. 1999), for single objects (Milky Way and Mrk 231; from Fixsen et al. 1999; van der Werf et al. 2010) and from simulations (Bournaud et al. 2015; Narayanan & Krumholz 2014). All SLEDs are normalised to the CO(5 − 4) flux observed in XID2028, for which we report an uncertainty of 0.08 Jy km s⁻¹ also accounting for the 10% flux calibration error. Lower panel: Same as the upper panel, but with the y-axis in logarithmic scale, so that differences in the CO(1 − 0) flux extrapolations can be appreciated.