Bound Gas, Dense Gas, and Star Formation: A Deceptively Simple Braid

Abstract

We apply gravity-based and density-based methodsto identify clouds in self-consistent numerical simulations of the star-forming, multiphase interstellar medium (ISM), and compare their properties and their global correlation with the star formation rate (SFR) over time. We study two Solar Neighborhood models at different resolution, and compare various other galactic conditions to study environmental dependence. The gravity-based method identifies bound objects, which have masses M ~ 10^3-10^5 Msun at densities n_H ~ 100-1000 per cc, and virial parameters alpha_v ~ 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 relatively high mass clouds (above 10^4-10^6 Msun) are generally unbound. This suggests that the traditional alpha_v is at best an approximate measure of boundedness in the ISM. Solar neighborhood clouds have internal turbulent motions increasing with size as sigma ~ 1km/s (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 e_ff ~0.4, across all environments. In the Solar Neighborhood, for n_H,min = 10-100 per cc, e_ff ~ 0.03 - 0.3, increasing with density threshold. Temporal correlation analysis between SFR(t) and aggregate mass M(n_H,min;t) at varying n_H,min shows that time delays to star formation are t_delay~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 - e_ff M/t_ff)/<SFR> is ~50%, due to the large amplitude variations and inherent stochasticity of the system. Our environmental dependence study has revealed a number of transitions between self-gravitating and non-self-gravitating gas occurring near densities which increase with the ambient density. We find these transitions in line width-size relations, alpha_v-M distributions, and most directly, in the typical density of bound objects. We find that e_ff for objects at fixed n_H,min decreases with higher ambient density, suggesting that star formation occurs relative to ambient density.