Left panel: L′(CO[1−0]) against the IR luminosity (8−1000 μm) showing a compilation of MS galaxies at 0 <z< 2.5 from the Sargent et al. (2014) work (Sa14; grey symbols), local ULIRGs from from So97, crosses SMGs from Bothwell et al. (2013; Bo13, filled brown squares), 1 <z< 4 QSOs with data from Riechers (2011) and Carilli & Walter (2013; R11/C&W13, light blue circles). The measurement for XID2028 obtained assuming r₃₁ = 0.7 is shown as a red circle. Obscured QSOs proposed to be in the transition phase presented in Aravena et al. (2008), Polletta et al. (2011), Feruglio et al. (2014) and the MIPS-selected sources from Yan et al. (2010) are also marked, as labelled. Right panel: inverse, integrated Kennicutt-Schmidt relation between SFR and molecular gas mass. The colour points correspond to the position of XID2028 with different CO-to-H₂ conversion factors: α_CO = 3.6 (top; blue), and α_CO = 0.8 (bottom; red). The green star shows the ISM mass inferred from the dust SED. All the values for XID2028 are slightly offset in the x-axis for clarity. Other points are taken from the same samples presented in the left panel. For the Bo13, R11/C&W13, and Ya10 samples we plot the median value with associated 16% and 84% percentiles. For the obscured QSOs all the authors used α_CO = 0.8. In both panels, the solid black line is the best-fit relation for MS galaxies, and the dashed line defines the locus of strong SB galaxies with approximately 15 times shorter depletion time (M_mol/SFR) than MS galaxies (Sargent et al. 2014).