How Galaxies Form Stars: The Connection between Local and Global Star Formation in Galaxy Simulations

Using a suite of isolated La. galaxy simulations, we show that global depletion times and star-forming gas mass fractions in simulated galaxies exhibit systematic and well-defined trends as a function of the local star formation efficiency per freefall time, epsilon(ff), strength of stellar feedback, and star formation threshold. We demonstrate that these trends can be reproduced and explained by a simple physical model of global star formation in galaxies. Our model is based on mass conservation and the idea of gas cycling between star-forming and non-star-forming states on certain characteristic timescales under the influence of dynamical and feedback processes. Both the simulation results and our model predictions exhibit two limiting regimes with rather different dependencies of global galactic properties on the local parameters. When epsilon(ff) is small and feedback is inefficient, the total star-forming mass fraction, f(sf), is independent of epsilon(ff) and the global depletion time, tau(dep), scales inversely with epsilon(ff). When epsilon(ff) is large or feedback is very efficient, these trends are reversed: f(sf) alpha epsilon(-1)(ff) and tau(dep) is independent of epsilon(ff) but scales linearly with the feedback strength. We also compare our results with the observed depletion times and mass fractions of star-forming and molecular gas and show that they provide complementary constraints on epsilon(ff) and the feedback strength. We show that useful constraints on epsilon(ff) can also be obtained using measurements of the depletion time and its scatter on different spatial scales.