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∼ 103 - 104,M⊙ at densities nH∼ 100, cm-3, and virial parameters α v ∼ 0.5-5. For clouds defined by a density threshold nH,min, the average virial parameter decreases, and the fraction of material that is genuinely bound increases, with increasing nH,min. Surprisingly, clouds defined by density thresholds can be unbound even when α v < 2, and high-mass clouds (104 - 106,M⊙) are generally unbound. This suggests that the traditional α v is at best an approximate measure of boundedness in the ISM. All clouds have internal turbulent motions increasing with size as, similar to observed relations. Bound structures comprise a small fraction of the total simulation mass and have a star formation efficiency per freefall time ϵff ∼ 0.4. For nH,min =10 - 100, cm-3, ϵff ∼ 0.03-0.3, increasing with density threshold. A temporal correlation analysis between SFR(t) and aggregate mass M(nH,mint) nH,min shows that time delays to star formation are tdelay∼ tff (nH,min). The correlation between SFR(t) and M(nH,mint) systematically tightens at higher nH,min. Considering moderate-density gas, selecting against high virial parameter clouds improves correlation with the SFR, consistent with previous work. Even at high nH,min, the temporal dispersion in is ∼50%, due to the large-amplitude variations and inherent stochasticity of the system.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science