Heidelberg Joint Astronomical Colloquium

Recent observations have been able to resolve the gas content and star formation rate (SFR) in nearby galaxies down to sub-kpc scales, leading to an empirical picture with three regimes of star formation. Outer galactic disks are dominated by atomic gas and have a steep dependence of the SFR on the gas content, and also show a correlation between SFR and the stellar content. Mid-disk regions are dominated by molecular gas and have a nearly linear relation between SFR and gas content. Starburst regions in galactic centers are molecule-dominated, and have a steep relation between SFR and gas. For all of these regimes, the gas supply divided by the dynamical time far exceeds the SFR, implying inefficient consumption of gas. To understand these empirical SFR relations, it is crucial to consider the ISM physics at scales less than the disk thickness. Rapid gas cooling and dissipation of turbulence demands constant injections of energy in order to maintain thermal and dynamical equilibrium in the ISM. Recently, we have developed theoretical models in which feedback self-regulates SFRs, adjusting to local environmental differences including the gravity imposed by the stellar disk. These models are in remarkably good agreement with observations in all three regimes of star formation, and have also been confirmed and calibrated using multiphase numerical hydrodynamic simulations. In this scenario, the reason that gas consumption so inefficient is that massive-star feedback is so efficient: stellar UV and expanding SN shells replenish the thermal and turbulent energy of the ISM in less than a dynamical time.