The model star-formation law. Upper panel: as a function of the total gas surface density. Middle panel: as a function of the molecular gas surface density. Lower panel: as a function of the atomic gas surface density. The solid lines show power-law fits with exponents 1.7 (total) and 1.2 (molecular). The solid line in the lower panel is there to guide the eye. Star formation is assumed to be proportional to the unobscured model UV emission (in arbitrary units). The molecular gas surface density is calculated by assuming that , where ρ is the total gas volume density. The atomic gas surface density is calculated by assuming that .