Cloud Properties and Correlations with Star Formation in Numerical
  Simulations of the Three-Phase ISM
  We apply gravity-based and density-based methods to identify clouds in
numerical simulations of the star-forming, three-phase interstellar medium
(ISM), and compare their properties and their global correlation with the star
formation rate over time. The gravity-based method identifies bound objects,
which have masses M ~ 10^3 - 10^4 M_solar at densities n_H ~ 100 cm^-3, and
traditional 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, at higher n_H,min.
Surprisingly, these clouds can be unbound even when alpha_v < 2, and high mass
clouds (10^4 - 10^6 M_solar) are generally unbound. This suggests that the
traditional alpha_v is at best an approximate measure of boundedness in the
ISM. All clouds have internal turbulent motions increasing with size as sigma ~
1 km/s(R/ pc)^1/2, similar to observed relations. Bound structures comprise a
small fraction of the total simulation mass, with star formation efficiency per
free-fall time epsilon_ff ~ 0.4. For n_H,min = 10 - 100 cm^-3, epsilon_ff ~
0.03 - 0.3, increasing with density. 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). Correlation between
SFR(t) and M(nH,min;t) systematically tightens at higher n_H,min. Considering
moderate-density gas, selecting against high virial parameter clouds improves
correlation with SFR, consistent with previous work. Even at high n_H,min, the
temporal dispersion in (SFR-epsilon_ff M/t_ff )/<SFR> is ~ 50%, due to the
large-amplitude variations and inherent stochasticity of the system.
