Left panel: observed ¹²CO SLED of AzTEC/C159 and other z > 4 SMGs for comparison: GN20 (Carilli et al. 2010), AzTEC-3 (Riechers et al. 2010), and J1000+0234 (Schinnerer et al. 2008). Gray lines show the expected trend for thermalization, i.e., LTE conditions. The dashed and solid color lines connect the data points to illustrate the shape of the different ¹² CO SLEDs. The line fluxes have been normalized to the ¹²CO(2 →1) line. Right panel: intrinsic ¹²CO SLED of AzTEC/C159. The observed flux densities for ¹²CO(2 →1) and ¹² CO(5 →4) have been corrected up by 1/0.7 and 1/0.8, respectively, to take into account the CMB effect (see Sect. 4.1). We also present the ¹²CO SLED of the Milky Way galactic center (Fixsen et al. 1999), MS disk-galaxies at z ~ 1.5 (Daddi et al. 2015), and a sample of SMGs (median of z = 2.2, Bothwell et al. 2013); similarly, we normalize all the line fluxes to the ¹²CO(2 → 1) line. Shaded regions show the dispersion of the ¹²CO SLEDs for different SMGs and disk galaxies samples.