Cold molecular gas surface density (Σ_H2) vs. IR luminosity surface density (Σ_LIR). Galaxy symbols are as in Fig. 5. The dotted black lines indicate the maximum luminosity from an instantaneous starburst using 100% (ϵ = 1) or 20% (ϵ = 0.2) of the available cold molecular gas (see Sect. 5.2.1). The orange solid line is the Eddington luminosity limit. For points above this line, the radiation pressure is stronger than gravity (Sect. 5.5). The dashed blue lines indicate the 1 and 10 Myr depletion times for reference assuming that the IR luminosity is produced by SF. The column density (N_H) axis is calculated from Σ_H2 assuming a uniform mass distribution (i.e., N_H = 2 × Σ_H2/m(H₂) where m(H₂) is the H₂ molecular weight).