Measurements of H \alpha , HI , and CO distributions in 61 normal spiral galaxies are combined with published far-infrared and CO observations of 36 infrared-selected starburst galaxies , in order to study the form of the global star formation law , over the full range of gas densities and star formation rates ( SFRs ) observed in galaxies .
The disk-averaged SFRs and gas densities for the combined sample are well represented by a Schmidt law with index N = 1.4 \pm 0.15 .
The Schmidt law provides a surprisingly tight parametrization of the global star formation law , extending over several orders of magnitude in SFR and gas density .
An alternative formulation of the star formation law , in which the SFR is presumed to scale with the ratio of the gas density to the average orbital timescale , also fits the data very well .
Both descriptions provide potentially useful “ recipes ” for modelling the SFR in numerical simulations of galaxy formation and evolution .