Astronomy Colloquium

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 an isothermal equation of state as an implicit model of feedback, quick production of molecular gas during gravitational collapse, and neglect of magnetic field. I examine and justify each of these assumptions in turn using extensive local models. Despite the simplicity of our assumptions, the global models quantitatively reproduce not only observed global and local 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 observed 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.