Control of Star Formation by Gravitational Instability and Supersonic Turbulence
I show how star formation depends on the nonlinear development of gravitational instability in galaxies, using high-resolution numerical models of star cluster formation in isolated and merging disk galaxies. Three main assumptions of these models are neglect of magnetic fields, an isothermal equation of state as an implicit model of feedback, and quick production of molecular gas during gravitational collapse. I examine and justify each of these assumptions in turn using extensive local models. These local models suggest that magnetostatic support does not dominate star formation, that the effective equation of state for interstellar gas is between isobaric and isothermal, and that molecular gas can form quickly in turbulent, gravitationally unstable gas. Despite the simplicity of our assumptions, the global models quantitatively reproduce not only observed global and local Schmidt laws (relationships between gas surface density and star formation rate), but also observed star formation thresholds in disk galaxies, properties of globular cluster systems in spiral and elliptical galaxies, and even the extended Magorrian relation between central massive object and bulge masses. Our results suggest that the dominant physical mechanism determining the star formation rate is the quantitative strength of gravitational instability.