  The star formation rate (SFR) in galactic disks depends on both the quantity
of available interstellar medium (ISM) gas and its physical state. Conversely,
the ISM's physical state depends on the SFR, because the "feedback" energy and
momentum injected by recently-formed massive stars is crucial to offsetting
losses from turbulent dissipation and radiative cooling. The ISM's physical
state also responds to the gravitational field that confines it, with increased
weight driving higher pressure. In a quasi-steady state, it is expected that
the mean total pressure of different thermal phases will match each other, that
the component pressures and total pressure will satisfy thermal and dynamical
equilibrium requirements, and that the SFR will adjust as needed to provide the
requisite stellar radiation and supernova feedback. The pressure-regulated,
feedback-modulated (PRFM) theory of the star-forming ISM formalizes these
ideas, leading to a prediction that the SFR per unit area, Sigma_SFR, will
scale nearly linearly with ISM weight W. In terms of large-scale gas surface
density Sigma, stellar plus dark matter density rho_sd, and effective ISM
velocity dispersion sigma_eff, an observable weight estimator is W~P_DE=pi G
Sigma^2/2+Sigma(2G rho_sd)^{1/2} sigma_eff, and this is predicted to match the
total midplane pressure P_tot. Using a suite of multiphase magnetohydrodynamic
simulations run with the TIGRESS computational framework, we test the
principles of the PRFM model and calibrate the total feedback yield Upsilon_tot
= P_tot/Sigma_SFR ~ 1000 km/s, as well as its components. We compare results
from TIGRESS to theory, previous numerical simulations, and observations,
finding excellent agreement.
