CO (left) and [CI] (right) SLEDs of MD94 (black) and other nearby and high-redshift systems. We indicate (when available) the SFR surface density in terms of the value measured in MD94 (Σ_SFR ≈ 2 M_⊙ yr⁻¹ kpc⁻²). The references for the ancillary data shown are the following: Mrk 231 (van der Werf et al. 2010), M82 (Kamenetzky et al. 2012), and Cloverleaf (Barvainis et al. 1997). In addition to these, we added BX610 (Brisbin et al. 2019; Aravena et al. 2014), average values for (U)LIRGs and starbursts (Israel et al. 2015), and the mean value for main-sequence galaxies at z ∼ 1 (Valentino et al. 2020b), and BzK main-sequence galaxies at z ∼ 1.5 (Daddi et al. 2015). The dashed dark gray line shows constant brightness temperature on the Rayleigh–Jeans scale (S ∝ ν²).