Comparison of the SFR surface density as a function of the molecular gas surface density for high-z MS and sub-mm galaxies and IC 4687. The red circle indicates the integrated measurement for IC 4687, as described in Sect. 5, using the Galactic α_CO factor. The error bars indicate systematic uncertainties due to the extinction correction in Σ_SFR (vertical) and a change in the α_CO factor from Galactic (assumed) to that considered for ULIRGs (horizontal; see Sect. 3.3). The green and red squares correspond to z ~ 1.2 and 2.2 MS SF galaxies from Tacconi et al. (2013) and BzK z ~ 1.5 galaxies from Daddi et al. (2010). For both datasets we used the Galactic α_CO factor. The blue, purple, and orange stars correspond to sub-millimeter galaxies at z ~ 2, 4.0, and 5.2 from Bothwell et al. (2010), Hodge et al. (2015), and Rawle et al. (2014), respectively, for which a ULIRG-like α_CO factor was applied. The dotted lines indicate constant log t_dep times.