Cloud Properties and Correlations with Star Formation in Self-consistent Simulations of the Multiphase ISM

Abstract

We apply gravity- and density-based methods to identify clouds in self-consistent numerical simulations of the star-forming, multiphase interstellar medium (ISM) and compare their properties and global correlation with the star formation rate (SFR) over time. The gravity-based method identifies bound objects, which have masses M similar to 10(3)-10(4) M-circle dot at densities n(H) similar to 100 cm(3), and virial parameters alpha(nu) similar to 0.5-5. For clouds defined by a density threshold n(H,min), the average virial parameter decreases, and the fraction of material that is genuinely bound increases, with increasing n(H,min).Surprisingly, clouds defined by density thresholds can be unbound even when alpha(v) < 2, and high-mass clouds (10(4)-10(6) M-circle dot) are generally unbound. This suggests that the traditional alpha(nu) is at best an approximate measure of boundedness in the ISM. All clouds have internal turbulent motions increasing with size as sigma similar to 1 km s(-1) (R/pc)(1/2), similar to observed relations. Bound structures comprise a small fraction of the total simulation mass and have a star formation efficiency per freefall time epsilon(ff) similar to 0.4. For n(H,min) = 10-100 cm(-3), E-ff similar to 0.03-0.3, increasing with density threshold. A temporal correlation analysis between SFR(t) and aggregate mass M(n(H,min); t) at varying n(H,min), min shows that time delays to star formation are t(delay) similar to t(ff) (n(H,min)). The correlation between SFR(t) and M(n(H,min); t) systematically tightens at higher n(H,min). Considering moderate-density gas, selecting against high virial parameter clouds improves correlation with the SFR, consistent with previous work. Even at high n(H,min), the temporal dispersion in (SFR - epsilon M-ff/t(ff))/< SFR > is similar to 50%, due to the large-amplitude variations and inherent stochasticity of the system.