The origin of physical variations in the star formation law

Observations of external galaxies and of local star-forming clouds in the Milky Way have suggested a variety of star formation laws, i.e. simple direct relations between the column density of star formation (Sigma(SFR): the amount of gas forming stars per unit area and time) and the column density of available gas (Sigma(gas)). Extending previous studies, we show that these different, sometimes contradictory relations for Milky Way clouds, nearby galaxies, and high-redshift discs and starbursts can be combined in one universal star formation law in which Sigma(SFR) is about 1 per cent of the local gas collapse rate, Sigma(gas)/t(ff), but a significant scatter remains in this relation. Using computer simulations and theoretical models, we find that the observed scatter may be primarily controlled by physical variations in the Mach number of the turbulence and by differences in the star formation efficiency. Secondary variations can be induced by changes in the virial parameter, turbulent driving and magnetic field. The predictions of our models are testable with observations that constrain both the Mach number and the star formation efficiency in Milky Way clouds, external disc and starburst galaxies at low and high redshift. We also find that reduced telescope resolution does not strongly affect such measurements when Sigma(SFR) is plotted against Sigma(gas)/t(ff).