Gas properties and implications for galactic star formation in numerical models of the turbulent, multiphase interstellar medium

Using numerical simulations of galactic disks that resolve scales from 1 to several hundred pc, we investigate dynamical properties of the multiphase interstellar medium (ISM) in which turbulence is driven by feedback from star formation. We focus on effects of H II regions by implementing a recipe for intense heating confined within dense, self-gravitating regions. Our models are two dimensional, representing radial-vertical slices through the disk, and include sheared background rotation of the gas, vertical stratification, heating and cooling to yield temperatures T ∼ 10-10⁴ K, and conduction that resolves thermal instabilities on our numerical grid. Each simulation evolves to reach a quasi-steady state, for which we analyze the time-averaged properties of the gas. In our suite of models, three parameters (the gas surface density Σ, the stellar volume density ρ_*, and the local angular rotation rate Ω) are separately controlled in order to explore environmental dependences. Among other statistical measures, we evaluate turbulent amplitudes, virial ratios, Toomre Q parameters including turbulence, and the mass fractions at different densities. We find that the dense gas (n>100 cm^-3) has turbulence levels similar to those observed in giant molecular clouds and virial ratios 1-2. Our models show that the Toomre Q parameter in the dense gas evolves to values near unity; this demonstrates self-regulation via turbulent feedback. We also test how the surface star formation rate Σ_SFR depends on Σ, ρ _*, and Ω. Under the assumption that the star formation rate (SFR) is proportional to the amount of gas at densities above a threshold n _th divided by the free-fall time at that threshold, we find that Σ_SFR Σ^1+p with 1 + p∼ 1.2-1.4 when n _th = 10² or 10³ cm^-3, consistent with observed Kennicutt-Schmidt relations. Estimates of SFRs based on large-scale properties (the orbital time, the Jeans time, or the free-fall time at the mean density within a scale height), however, depart from rates computed using the measured amount of dense gas, indicating that resolving the ISM structure in galactic disks is necessary to obtain accurate predictions of the SFR.