CO excitation ladder. Left: average CO SLEDs of our samples compared with results from the literature. Each SLED is normalized to the mean CO (2 − 1) flux of main-sequence galaxies at z = 1.25. Blue filled circles and open black squares indicate the mean SLEDs of galaxies on the main sequence, starbursts (ΔMS ≥ 3.5), and extreme starbursts (ΔMS ≥ 7), as labeled. The literature samples include the BzK galaxies from D15 (open light blue squares); the average SMGs from Bothwell et al. (2013) (filled green stars) and the SPT-selected ones from Spilker et al. (2014) (open golden stars); high-redshift QSOs from Carilli & Walter (2013) (open gray diamonds); local ULIRGs from Papadopoulos et al. (2012) (red filled circles); the inner disk of the Milky Way from Fixsen et al. (1999) (open pink triangles). The upper limits are at 3σ significance. The solid line shows the line ratios for a fully thermalized case. The gray shaded area marks the model by Narayanan & Krumholz (2014) for unresolved observations within Σ_SFR = 1−10 M_⊙ yr⁻¹ kpc⁻². The long dashed line indicates the simulations from Bournaud et al. (2015). The dotted line traces the empirical model by Papadopoulos et al. (2012). The dashed-dotted line points at the analytical model by Vollmer et al. (2017). Right: mean CO SLEDs for a subsample of objects with detections of CO (2 − 1), CO (5 − 4), and estimates of L_IR and FIR sizes, split at the median value of Σ_SFR. Light empty and dark filled red circles indicate the low and high Σ_SFR subsamples. Both SLEDs are normalized to the CO (2 − 1) flux of the main-sequence sample as in the left panel.