Gas Properties and Implications for Galactic Star Formation in Numerical Models of the Turbulent, Multiphase ISM
Abstract
Using numerical simulations of galactic disks resolving scales from ~1 to several hundred pc, we investigate dynamical properties of the multiphase ISM with turbulence driven by star formation feedback. We focus on HII region effects by applying intense heating in dense, self-gravitating regions. Our models are two-dimensional radial-vertical slices through the disk, and include sheared background rotation, vertical stratification, heating and cooling to yield temperatures T~10-104K, and thermal conduction. We separately vary the gas surface density Sigma, the stellar volume density rho*, and the local angular rotation rate Omega to explore environmental dependencies, and analyze the steady-state properties of each model. Among other statistics, 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 observed GMCs and virial ratios ~1-2. The Toomre Q parameter in dense gas reaches near unity, demonstrating self-regulation via turbulent feedback. We also test how the star formation rate SigmaSFR depends on Sigma, rho*, and Omega. Under the assumption that the star formation rate is proportional to the mass at densities above nth divided by the free-fall time at that threshold, we find that SigmaSFR varies as Sigma(1+p) with 1+p ~ 1.2-1.4 when nth=102 or 103 cm-3, consistent with observations. Estimated star formation rates based on large-scale properties (the orbital time, the Jeans time, or the free-fall time at the vertically-averaged density) however depart from rates computed using the dense gas mass, indicating that resolving the ISM structure in galactic disks at scales <<H is necessary for accurate predictions of the star formation rate.