Spiral Shocks, Cooling, and the Origin of Star Formation Rates

Abstract

We have performed the first numerical simulations to resolve the star formation process on sub-parsec scales, whilst also following the dynamics of the interstellar medium (ISM) on galactic scales. The warm low density ISM gas flows into the spiral arms where orbit crowding produces the shock formation of dense clouds, held together temporarily by their external pressure. Cooling allows the gas to be compressed to sufficiently high densities that local regions collapse under their own gravity and form stars. The star formation rates follow a Schmidt-Kennicutt $\varSigma _{\mathrm{SFR}} \propto \varSigma _{\mathrm{gas}}^{1.4}$ type relation with the local surface density of gas while following a linear relation with the dense cold gas. Cooling is the primary driver of star formation and the star formation rates as it determines the amount of cold gas available for gravitational collapse.