Star Formation By Demand
March 22, 2012
Abstract: When averaged over large scales, star formation rates (SFRs) are related to the gaseous content of disk galaxies by empirical Kennicutt-Schmidt (KS) laws, and are also correlated with stellar and dark matter content. Outer galactic disks are primarily atomic and have steep KS laws. Mid-disks are dominated by molecular clouds and have nearly linear KS laws. Starburst regions (in ULIRGs and high-z galaxies) are molecule-dominated and have steep KS law. Recent surveys have confirmed that the consumption of gas is highly inefficient in all regimes: the gas depletion timescales far exceed both global and local dynamical timescales. I will argue that the observed star formation rate scalings — and the low gas consumption efficiency — are a consequence of ISM self-regulation mediated by energy feedback from massive stars. In models we have recently developed, SFRs evolve to meet the demands of the local galactic environment. In equilibrium, ISM heating balances cooling, total pressure balances gravity, and turbulent driving balances dissipation. These simple models are in remarkably good agreement with observations in all three regimes of star formation, and have been confirmed and calibrated using multiphase numerical hydrodynamic simulations. Because stars are so efficient at producing energy, the ISM’s demands can be met with only slow depletion of a galaxy’s gaseous fuel.