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% 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.
