Astronomy Colloquium

  Galaxies are long-lived systems.  Disk galaxies like the Milky Way retain a massive reservoir of atomic and molecular gas over billions of years, converting it to stars only slowly.  Because gas consumption timescales far exceed both global and local dynamical timescales, star formation is often characterized as being highly inefficient. The perspective shifts if we consider the effects of star formation on the interstellar medium (ISM) rather than its causes: without star formation feedback, the ISM could not exist as we know it.  Rapid losses of thermal energy by radiative cooling and turbulent energy by dissipation must be constantly replenished to maintain the observed state of the ISM, and feedback from short-lived massive stars -- including UV radiation and supernova blasts -- is crucial. In this talk, I will discuss theory and numerical hydrodynamic/RHD simulations that quantify the physics of feedback and star formation self-regulation at a range of scales.  We find that radiation forces can be important to ejecting gas and limiting the efficiency of individual star-forming clouds. However, supernovae play the most important role in the ISM overall, because the momentum injected by Sedov-Taylor blast waves is an order of magnitude greater than other source terms.  Resolved simulations show that each supernova blast robustly provides momentum ~ 1-4 e5 Msun km/s to the ISM.  This momentum yield is just what is required to explain ISM properties and star formation rates as observed in diverse galactic environments, with the self-regulation model providing a unified theoretical framework and quantitative explanation for both Kennicutt-Schmidt and pressure-based empirical star formation laws.