Physical drivers of the CO excitation. L′ [K km s⁻¹ pc²] luminosity ratio between CO (5 − 4) and CO (2 − 1) as a function of ⟨U⟩ from SED modeling (left), SFE (center), and Σ_SFR (right). Symbols are color-coded as labeled (see Sect. 3.6 for references), where our samples of main-sequence and starburst galaxies at z ∼ 1.3 are marked by blue filled circles and open black squares, respectively. The solid blue line and the shaded area mark the best-fit model from the linear regression analysis and its 95% confidence interval (see Table 3 for the parameters). The solid black line shows the best-fit model from D15, based on the average values for local spirals, ULIRGs, and BzKs at z = 1.5 (large open red circle, filled red circle, and open blue triangle, respectively). The short- and long-dashed black lines in the right panel indicate the predicted trends from the simulations by Narayanan & Krumholz (2014) and Bournaud et al. (2015), respectively.