Relation between the star formation rate surface density and the molecular gas surface density at scales of ≃100 pc. Different symbols indicate different galaxies. The solid black line shows the best fit obtained by Bigiel et al. (2008) (scaled to the same X_CO conversion factor) for the Heracles sample of nearby galaxies using star formation rates derived from Hα+24μm flux surface densities. The dotted lines show lines of constant star formation efficiency, indicating the level of Σ_SFR needed to consume 1%, 10%, and 100% of the gas reservoir (including helium) in 10⁸ years, corresponding to depletion times of 10⁸, 10⁹, and 10¹⁰ years.