arXiv subjects

Eve C. Ostriker

Publications and source records attributed to Eve C. Ostriker.

At least 73 records · Page 4Linked to original sources

arXiv2021

Star Formation Efficiency and Dispersal of Giant Molecular Clouds with UV Radiation Feedback: Dependence on Gravitational Boundedness and Magnetic Fields

Molecular clouds are supported by turbulence and magnetic fields, but quantifying their influence on cloud lifecycle and star formation efficiency (SFE) remains an open question. We perform radiation MHD simulations of star-forming giant molecular clouds (GMCs) with UV radiation feedback, in which the propagation of UV radiation via ray-tracing is coupled to hydrogen photochemistry. We consider 10 GMC models that vary in either initial virial parameter ($1\leα_{v,0}\le 5$) or dimensionless mass-to-magnetic flux ratio (0.5-8 and $\infty$); the initial mass $10^5M_{\odot}$ and radius 20pc are fixed. Each model is run with five different initial turbulence realizations. In most models, the duration of star formation and the timescale for molecular gas removal (primarily by photoevaporation) are 4-8Myr. Both the final SFE ($ε_*$) and time-averaged SFE per freefall time ($ε_{ff}$) are reduced by strong turbulence and magnetic fields. The median $ε_*$ ranges between 2.1% and 9.5%. The median $ε_{ff}$ ranges between 1.0% and 8.0% and anticorrelates with $α_{v,0}$, in qualitative agreement with previous analytic theory and simulations. However, the time-dependent $α_{v}(t)$ and $ε_{ff,obs}(t)$ based on instantaneous gas properties and cluster luminosity are positively correlated due to rapid evolution, making observational validation of star formation theory difficult. Our median $ε_{ff,obs}(t)\approx$ 2% is similar to observed values. We show that the traditional virial parameter estimates the true gravitational boundedness within a factor of 2 on average, but neglect of magnetic support and velocity anisotropy can sometimes produce large departures. Magnetically subcritical GMCs are unlikely to represent sites of massive star formation given their unrealistic columnar outflows, prolonged lifetime, and low escape fraction of radiation.

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arXiv2020

Simulated Observations of Multiphase Galactic Winds

Supernova-driven galactic winds are multiphase streams of gas that are often observed flowing at a range of velocities out of star-forming regions in galaxies. In this study, we use high resolution 3D simulations of multiphase galactic winds modeled with the hydrodynamics code Cholla to investigate the connection between numerical studies and observations. Using a simulated interaction between a hot $T\sim 10^{7}\,\rm K$ supernova-driven wind and a cool $T\sim 10^{4}\,\rm K$ cloud of interstellar material, we create mock observables, including the optical depth and covering fraction of six commonly observed ions (Si II, C II, Si IV, C IV, N V, and O VI) as a function of gas velocity. We compare our mock observables to surveys of galactic winds in the literature, finding good agreement with velocities and profiles of the low ions. We then compute "empirical" values for the optical depth and covering fraction following observational techniques, and compare them to the values calculated directly from the simulation data. We find that the empirically computed values tend to underestimate the "true" value of $τ$ for ions with high optical depth, and overestimate the "true" value of $τ$ for ions with low optical depth, relative to the simulated data.

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arXiv2020

Radiative Supernova Remnants and Supernova Feedback

Supernova (SN) explosions are a major feedback mechanism regulating star formation in galaxies through their momentum input. We review the observations of SNRs in radiative stages in the Milky Way to validate the theoretical results on the momentum/energy injection from a single SN explosion. For seven SNRs where we can observe fast-expanding, atomic radiative shells, we show that the shell momentum inferred from HI 21 cm line observations is in the range of (0.5--4.5)$\times 10^5$ $M_\odot$ km s$^{-1}$. In two SNRs (W44 and IC 443), shocked molecular gas with momentum comparable to that of the atomic SNR shells has been also observed. We compare the momentum and kinetic/thermal energy of these seven SNRs with the results from 1D and 3D numerical simulations. The observation-based momentum and kinetic energy agree well with the expected momentum/energy input from an SN explosion of $\sim 10^{51}$ erg. It is much more difficult to use data/model comparisons of thermal energy to constrain the initial explosion energy, however, due to rapid cooling and complex physics at the hot/cool interface in radiative SNRs. We discuss the observational and theoretical uncertainties of these global parameters and explosion energy estimates for SNRs in complex environments.

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arXiv2020

First results from SMAUG: Insights into star formation conditions from spatially-resolved ISM properties in TNG50

Physical and chemical properties of the interstellar medium (ISM) at sub-galactic ($\sim$kpc) scales play an indispensable role in controlling the ability of gas to form stars. As part of the SMAUG (Simulating Multiscale Astrophysics to Understand Galaxies) project, in this paper, we use the TNG50 cosmological simulation to explore the physical parameter space of 8 resolved ISM properties in star-forming regions to constrain the areas of this hyperspace over which most star-forming environments exist. We deconstruct our simulated galaxies spanning a wide range of mass (M$_\star = 10^{7-11}$ M$_\odot$) and redshift ($0 \leq z \leq 3$) into kpc-sized regions, and statistically analyze the gas/stellar surface densities, gas metallicity, vertical stellar velocity dispersion, epicyclic frequency and dark-matter volumetric density representative of each region in the context of their star formation activity and galactic environment (radial galactocentric location). By examining the star formation rate (SFR) weighted distributions of these properties, we show that stars primarily form in two spatially distinct environmental regimes, which are brought about by an underlying bi-component radial SFR surface density profile in galaxies. We examine how the relative prominence of these two regimes depends on host galaxy mass and cosmic time. We also compare our findings with those from integral field spectroscopy observations and achieve a good overall agreement. Further, using dimensionality reduction, we characterise the aforementioned hyperspace to reveal a high-degree of multicollinearity in relationships amongst ISM properties that drive the distribution of star formation at kpc-scales. Based on this, we show that a reduced 3D representation underpinned by a multi-variate radius relationship is sufficient to capture most of the variance in the original 8D space.

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arXiv2020

A Framework for Multiphase Galactic Wind Launching using TIGRESS

Galactic outflows have density, temperature, and velocity variations at least as large as that of the multiphase, turbulent interstellar medium (ISM) from which they originate. We have conducted a suite of parsec-resolution numerical simulations using the TIGRESS framework, in which outflows emerge as a consequence of interaction between supernovae (SNe) and the star-forming ISM. The outflowing gas is characterized by two distinct thermal phases, cool (T<10^4 K) and hot (T>10^6 K), with most mass carried by the cool phase and most energy and newly-injected metals carried by the hot phase. Both components have a broad distribution of outflow velocity, and especially for cool gas this implies a varying fraction of escaping material depending on the halo potential. Informed by the TIGRESS results, we develop straightforward analytic formulae for the joint probability density functions (PDFs) of mass, momentum, energy, and metal loading as distributions in outflow velocity and sound speed. The model PDFs have only two parameters, SFR surface density Σ_SFR and the metallicity of the ISM, and fully capture the behavior of the original TIGRESS simulation PDFs over Σ_SFR~(10^{-4},1)M_sun/kpc^2/yr. Employing PDFs from resolved simulations will enable galaxy formation subgrid model implementations with wind velocity and temperature (as well as total loading factors) that are based on theoretical predictions rather than empirical tuning. This is a critical step to incorporate advances from TIGRESS and other high-resolution simulations in future cosmological hydrodynamics and semi-analytic galaxy formation models. We release a python package to prototype our model and to ease its implementation.

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arXiv2020

The environmental dependence of the X_CO conversion factor

CO is the most widely used observational tracer of molecular gas. The observable CO luminosity is translated to H_2 mass via a conversion factor, X_CO, which is a source of uncertainty and bias. Despite variations in X_CO, the empirically-determined solar neighborhood value is often applied across different galactic environments. To improve understanding of X_CO, we employ 3D magnetohydrodynamics simulations of the interstellar medium (ISM) in galactic disks with a large range of gas surface densities, allowing for varying metallicity, far-ultraviolet (FUV) radiation, and cosmic ray ionization rate (CRIR). With the TIGRESS simulation framework we model the three-phase ISM with self-consistent star formation and feedback, and post-process outputs with chemistry and radiation transfer to generate synthetic CO(1--0) and (2--1) maps. Our models reproduce the observed CO excitation temperatures, line-widths, and line ratios in nearby disk galaxies. X_CO decreases with increasing metallicity, with a power-law slope of -0.8 for the (1--0) line and -0.5 for the (2--1) line. X_CO also decreases at higher CRIR, and is insensitive to the FUV radiation. As density increases, X_CO first decreases due to increasing excitation temperature, and then increases when the emission is fully saturated. We provide fits between X_CO and observable quantities such as the line ratio, peak antenna temperature, and line brightness, which probe local gas conditions. These fits, which allow for varying beam size, may be used in observations to calibrate out systematic biases. We also provide estimates of the CO-dark H_2 fraction at different gas surface densities, observational sensitivities, and beam sizes.

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arXiv2020

Molecular Gas Properties on Cloud Scales Across the Local Star-forming Galaxy Population

Using the PHANGS-ALMA CO (2-1) survey, we characterize molecular gas properties on ${\sim}$100 pc scales across 102,778 independent sightlines in 70 nearby galaxies. This yields the best synthetic view of molecular gas properties on cloud scales across the local star-forming galaxy population obtained to date. Consistent with previous studies, we observe a wide range of molecular gas surface densities (3.4 dex), velocity dispersions (1.7 dex), and turbulent pressures (6.5 dex) across the galaxies in our sample. Under simplifying assumptions about sub-resolution gas structure, the inferred virial parameters suggest that the kinetic energy of the molecular gas typically exceeds its self-gravitational binding energy at ${\sim}$100 pc scales by a modest factor (1.3 on average). We find that the cloud-scale surface density, velocity dispersion, and turbulent pressure (1) increase towards the inner parts of galaxies, (2) are exceptionally high in the centers of barred galaxies (where the gas also appears less gravitationally bound), and (3) are moderately higher in spiral arms than in inter-arm regions. The galaxy-wide averages of these gas properties also correlate with the integrated stellar mass, star formation rate, and offset from the star-forming main sequence of the host galaxies. These correlations persist even when we exclude regions with extraordinary gas properties in galaxy centers, which contribute significantly to the inter-galaxy variations. Our results provide key empirical constraints on the physical link between molecular cloud populations and their galactic environment.

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arXiv2020

First results from SMAUG: Characterization of Multiphase Galactic Outflows from a Suite of Local Star-Forming Galactic Disk Simulations

Large scale outflows in star-forming galaxies are observed to be ubiquitous, and are a key aspect of theoretical modeling of galactic evolution in a cosmological context, the focus of the SMAUG (Simulating Multiscale Astrophysics to Understand Galaxies) project. Gas blown out from galactic disks, similar to gas within galaxies, consists of multiple phases with large contrasts of density, temperature, and other properties. To study multiphase outflows as emergent phenomena, we run a suite of ~pc-resolution local galactic disk simulations using the TIGRESS framework. Explicit modeling of the interstellar medium (ISM), including star formation and self-consistent radiative heating plus supernova feedback, regulates ISM properties and drives the outflow. We investigate the scaling of outflow mass, momentum, energy, and metal loading factors with galactic disk properties, including star formation rate (SFR) surface density (Σ_SFR~10^{-4}-1 M_sun/kpc^2/yr), gas surface density (~1-100 M_sun/pc^2), and total midplane pressure (or weight) (~10^3-10^6 k_B cm^{-3} K). The main components of outflowing gas are mass-delivering cool gas (T~10^4 K) and energy/metal-delivering hot gas (T~10^6 K). Cool mass outflow rates measured at outflow launch points (one or two scale heights) are 1-100 times the SFR (decreasing with Σ_SFR), although in massive galaxies most mass falls back due to insufficient outflow velocity. The hot galactic outflow carries mass comparable to 10% of the SFR, together with 10-20% of the energy and 30-60% of the metal mass injected by SN feedback. The characteristic outflow velocities of both phases scale very weakly with SFR, as v_out \propto Σ_SFR^{0.1~0.2}, consistent with observations. Importantly, our analysis demonstrates that in any physically-motivated cosmological wind model, it is crucial to include at least two distinct thermal wind components.

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arXiv2020

The Impact of Type Ia Supernovae in Quiescent Galaxies: I. Formation of the Multiphase Interstellar medium

A cool phase of the interstellar medium has been observed in many giant elliptical galaxies, but its origin remains unclear. We propose that uneven heating from Type Ia supernovae (SNe Ia), together with radiative cooling, can lead to the formation of the cool phase. The basic idea is that since SNe Ia explode randomly, gas parcels which are not directly heated by SN shocks will cool, forming multiphase gas. We run a series of idealized high-resolution numerical simulations, and find that cool gas develops even when the overall SNe heating rate $H$ exceeds the cooling rate $C$ by a factor as large as 1.4. We also find that the time for multiphase gas development depends on the gas temperature. When the medium has a temperature $T = 3\times 10^6$ K, the cool phase forms within one cooling time \tc; however, the cool phase formation is delayed to a few times \tc\ for higher temperatures. The main reason for the delay is turbulent mixing. Cool gas formed this way would naturally have a metallicity lower than that of the hot medium. For constant $H/C$, there is more turbulent mixing for higher temperature gas. We note that this mechanism of producing cool gas cannot be captured in cosmological simulations, which usually fail to resolve individual SN remnants.

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arXiv2020

The Impact of Type Ia Supernovae in Quiescent Galaxies: II. Energetics and Turbulence

Type Ia supernovae (SNe Ia) provide unique and important feedback in quiescent galaxies, but their impact has been underappreciated. In this paper, we analyze a series of high-resolution simulations to examine the energetics and turbulence of the medium under SNe Ia. We find that when SN remnants are resolved, their effects differ distinctly from a volumetric heating term, as is commonly assumed in unresolved simulations. First, the net heating is significantly higher than expected, by 30$\pm$10\% per cooling time. This is because a large fraction of the medium is pushed into lower densities which cool inefficiently. Second, the medium is turbulent; the root-mean-squared (RMS) velocity of the gas to 20-50 km s$^{-1}$ on a driving scale of tens of parsec. The velocity field of the medium is dominated by compressional modes, which are larger than the solenoidal components by a factor of 3-7. Third, the hot gas has a very broad density distribution. The ratio between the density fluctuations and the RMS Mach number, parameterized as $b$, is 2-20. This is in contrast to previous simulations of turbulent media, which have found $b\lesssim$ 1. The reason for the difference is mainly caused by the \textit{localized} heating of SNe Ia, which creates a large density contrast. Last, the typical length scale of a density fluctuation grows with time, forming increasingly larger bubbles and filamentary ridges. These underlying density fluctuations need to be included when X-ray observations are interpreted.

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arXiv2020

Diffuse Ionized Gas in Simulations of Multiphase, Star-Forming Galactic Disks

It has been hypothesized that photons from young, massive star clusters are responsible for maintaining the ionization of diffuse warm ionized gas seen in both the Milky Way and other disk galaxies. For a theoretical investigation of the warm ionized medium (WIM), it is crucial to solve radiation transfer equations where the ISM and clusters are modeled self-consistently. To this end, we employ a Solar neighborhood model of TIGRESS, a magnetohydrodynamic simulation of the multiphase, star-forming ISM, and post-process the simulation with an adaptive ray tracing method to transfer UV radiation from star clusters. We find that the WIM volume filling factor is highly variable, and sensitive to the rate of ionizing photon production and ISM structure. The mean WIM volume filling factor rises to ~0.15 at |z|~1 kpc. Approximately half of ionizing photons are absorbed by gas and half by dust; the cumulative ionizing photon escape fraction is 1.1%. Our time-averaged synthetic H$α$ line profile matches WHAM observations on the redshifted (outflowing) side, but has insufficient intensity on the blueshifted side. Our simulation matches the Dickey-Lockman neutral density profile well, but only a small fraction of snapshots have high-altitude WIM density consistent with Reynolds Layer estimates. We compute a clumping correction factor C = /sqrt ~0.2 that is remarkably constant with distance from the midplane and time; this can be used to improve estimates of ionized gas mass and mean electron density from observed H$α$ surface brightness profiles in edge-on galaxies.

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arXiv2020

Local Simulations of Spiral Galaxies with the TIGRESS Framework: I. Star Formation and Arm Spurs/Feathers

Spiral arms greatly affect gas flows and star formation in disk galaxies. We use local three-dimensional simulations of the vertically-stratified, self-gravitating, differentially-rotating, interstellar medium (ISM) subject to a stellar spiral potential to study the effects of spiral arms on star formation and formation of arm spurs/feathers. We adopt the TIGRESS framework of Kim & Ostriker (2017) to handle radiative heating and cooling, star formation, and ensuing supernova (SN) feedback. We find that more than 90% of star formation takes place in spiral arms, but the global star formation rate (SFR) in models with spiral arms is enhanced by less than a factor of 2 compared to the no-arm counterpart. This results from a quasi-linear relationship between the SFR surface density Sigma_SFR and the gas surface density Sigma, and supports the picture that spiral arms do not trigger star formation but rather concentrate star-forming regions. Correlated SN feedback produces gaseous spurs/feathers downstream from arms in both magnetized and unmagnetized models. These spurs/feathers are short-lived and have magnetic fields parallel to their length, in contrast to the longer-lived features with perpendicular magnetic fields induced by gravitational instability. SN feedback drives the turbulent component of magnetic fields, with the total magnetic field strength sublinearly proportional to Sigma. The total midplane pressure varies by a factor of ~10 between arm and interarm regions but agrees locally with the total vertical ISM weight, while Sigma_SFR is locally consistent with the prediction of pressure-regulated, feedback-modulated theory.

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arXiv2020

Self-gravitating Filament Formation from Shocked Flows: Velocity Gradients across Filaments

In typical environments of star-forming clouds, converging supersonic turbulence generates shock-compressed regions, and can create strongly-magnetized sheet-like layers. Numerical MHD simulations show that within these post-shock layers, dense filaments and embedded self-gravitating cores form via gathering material along the magnetic field lines. As a result of the preferred-direction mass collection, a velocity gradient perpendicular to the filament major axis is a common feature seen in simulations. We show that this prediction is in good agreement with recent observations from the CARMA Large Area Star Formation Survey (CLASSy), from which we identified several filaments with prominent velocity gradients perpendicular to their major axes. Highlighting a filament from the northwest part of Serpens South, we provide both qualitative and quantitative comparisons between simulation results and observational data. In particular, we show that the dimensionless ratio $C_v \equiv {Δv_h}^2/(GM/L)$, where $Δv_h$ is half of the observed perpendicular velocity difference across a filament, and $M/L$ is the filament's mass per unit length, can distinguish between filaments formed purely due to turbulent compression and those formed due to gravity-induced accretion. We conclude that the perpendicular velocity gradient observed in the Serpens South northwest filament can be caused by gravity-induced anisotropic accretion of material from a flattened layer. Using synthetic observations of our simulated filaments, we also propose that a density-selection effect may explain observed subfilaments (one filament breaking into two components in velocity space) as reported in Dhabal et al. (2018).

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arXiv2020

Multiphase Gas and the Fractal Nature of Radiative Turbulent Mixing Layers

A common situation in galactic and intergalactic gas involves cold dense gas in motion relative to hot diffuse gas. Kelvin-Helmholtz instability creates a turbulent mixing layer and populates the intermediate-temperature phase, which often cools rapidly. The energy lost to cooling is balanced by the advection of hot high enthalpy gas into the mixing layer, resulting in growth and acceleration of the cold phase. This process may play a major role in determining the interstellar medium and circumgalactic medium phase structure, and accelerating cold gas in galactic winds and cosmic filaments. Cooling in these mixing layers occurs in a thin corrugated sheet, which we argue has an area with fractal dimension $D=5/2$ and a thickness that adjusts to match the hot phase mixing time to the cooling time. These cooling sheet properties form the basis of a new model for how the cooling rate and hot gas inflow velocity depend on the size $L$, cooling time $t_{\rm cool}$, relative velocity $v_{\rm rel}$, and density contrast $ρ_{\rm cold}/ρ_{\rm hot}$ of the system. Entrainment is expected to be enhanced in environments with short $t_{\rm cool}$, large $v_{\rm rel}$, and large $ρ_{\rm cold}/ρ_{\rm hot}$. Using a large suite of three dimensional hydrodynamic simulations, we demonstrate that this fractal cooling layer model accurately captures the energetics and evolution of turbulent interfaces and can therefore be used as a foundation for understanding multiphase mixing with strong radiative cooling.

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arXiv2020

The Physical Nature of Starburst-Driven Galactic Outflows

We present the fourth of the Cholla Galactic OutfLow Simulations suite (CGOLS). Using a physically-motivated prescription for clustered supernova feedback, we successfully drive a multiphase outflow from a disk galaxy. The high resolution ($< 5\,\mathrm{pc}$) across a relatively large domain ($20\,\mathrm{kpc}$) allows us to capture the hydrodynamic mixing and dynamical interactions between the hot and cool ($T \sim 10^4\,\mathrm{K}$) phases in the outflow, which in turn leads to direct evidence of a qualitatively new mechanism for cool gas acceleration in galactic winds. We show that mixing of momentum from the hot phase to the cool phase accelerates the cool gas to $800\,\mathrm{km}\,\mathrm{s}^{-1}$ on kpc scales, with properties inconsistent with the physical models of ram pressure acceleration or with bulk cooling from the hot phase. The mixing process also affects the hot phase, modifying its radial profiles of temperature, density, and velocity from the expectations of radial supersonic flow. This mechanism provides a physical explanation for the high velocity, blue shifted, low ionization absorption lines often observed in the spectra of starburst and high redshift galaxies.

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arXiv2020

Dynamical Equilibrium in the Molecular ISM in 28 Nearby Star-Forming Galaxies

We compare the observed turbulent pressure in molecular gas, $P_\mathrm{turb}$, to the required pressure for the interstellar gas to stay in equilibrium in the gravitational potential of a galaxy, $P_\mathrm{DE}$. To do this, we combine arcsecond resolution CO data from PHANGS-ALMA with multi-wavelength data that traces the atomic gas, stellar structure, and star formation rate (SFR) for 28 nearby star-forming galaxies. We find that $P_\mathrm{turb}$ correlates with, but almost always exceeds the estimated $P_\mathrm{DE}$ on kiloparsec scales. This indicates that the molecular gas is over-pressurized relative to the large-scale environment. We show that this over-pressurization can be explained by the clumpy nature of molecular gas; a revised estimate of $P_\mathrm{DE}$ on cloud scales, which accounts for molecular gas self-gravity, external gravity, and ambient pressure, agrees well with the observed $P_\mathrm{turb}$ in galaxy disks. We also find that molecular gas with cloud-scale ${P_\mathrm{turb}}\approx{P_\mathrm{DE}}\gtrsim{10^5\,k_\mathrm{B}\,\mathrm{K\,cm^{-3}}}$ in our sample is more likely to be self-gravitating, whereas gas at lower pressure appears more influenced by ambient pressure and/or external gravity. Furthermore, we show that the ratio between $P_\mathrm{turb}$ and the observed SFR surface density, $Σ_\mathrm{SFR}$, is compatible with stellar feedback-driven momentum injection in most cases, while a subset of the regions may show evidence of turbulence driven by additional sources. The correlation between $Σ_\mathrm{SFR}$ and kpc-scale $P_\mathrm{DE}$ in galaxy disks is consistent with the expectation from self-regulated star formation models. Finally, we confirm the empirical correlation between molecular-to-atomic gas ratio and kpc-scale $P_\mathrm{DE}$ reported in previous works.

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arXiv2019

Kinematics and Dynamics of Multiphase Outflows in Simulations of the Star-Forming Galactic ISM

Galactic outflows produced by stellar feedback are known to be multiphase in nature. Both observations and simulations indicate that the material within several kpc of galactic disk mid-planes consists of warm clouds embedded within a hot wind. A theoretical understanding of the outflow phenomenon, including both winds and fountain flows, requires study of the interactions among thermal phases. We develop a method to quantify these interactions via measurements of mass, momentum, and energy flux exchanges using temporally and spatially averaged quantities and conservation laws. We apply this method to a star-forming ISM MHD simulation based on the TIGRESS framework, for Solar neighbourhood conditions. To evaluate the extent of interactions among the phases, we first examine the validity of the ``ballistic model,'' which predicts trajectories of the warm phase ($5050\,\rm{K} 5\times10^5\,\rm{K}$) to the warm phase. The large energy flux from the hot outflow that is transferred to the warm and intermediate phases is quickly radiated away. A simple interaction model implies an effective warm cloud size in the fountain flow of a few 100~pc, showing that warm-hot flux exchange mainly involves a few large clouds rather than many small ones.

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arXiv2019

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 )/ is ~ 50%, due to the large-amplitude variations and inherent stochasticity of the system.

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