Star Formation By Demand

Eve Ostriker (U. of Maryland)

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.