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Register a new userNon-universal star formation efficiency in turbulent ISM
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We present a study of a star formation prescription in which star formation efficiency depends on local gas density and turbulent velocity dispersion, as suggested by direct simulations of SF in turbulent giant molecular clouds (GMCs). We test the model using a simulation of an isolated Milky Way-sized galaxy with a self-consistent treatment of turbulence on unresolved scales. We show that this prescription predicts a wide variation of local star formation efficiency per free-fall time, $epsilon_{rm ff} sim 0.1 - 10%$, and gas depletion time, $t_{rm dep} sim 0.1 - 10$ Gyr. In addition, it predicts an effective density threshold for star formation due to suppression of $epsilon_{rm ff}$ in warm diffuse gas stabilized by thermal pressure. We show that the model predicts star formation rates in agreement with observations from the scales of individual star-forming regions to the kiloparsec scales. This agreement is non-trivial, as the model was not tuned in any way and the predicted star formation rates on all scales are determined by the distribution of the GMC-scale densities and turbulent velocities $sigma$ in the cold gas within the galaxy, which is shaped by galactic dynamics. The broad agreement of the star formation prescription calibrated in the GMC-scale simulations with observations, both gives credence to such simulations and promises to put star formation modeling in galaxy formation simulations on a much firmer theoretical footing.
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The Taffy system (UGC 12914/15) consists of two massive spiral galaxies which had a head-on collision about 20 Myr ago. New sensitive, high-resolution CO(1-0) observations of the Taffy system with the IRAM PdBI are presented. About 25% of the total interferometric CO luminosity stems from the bridge region. Assuming a Galactic N(H2)/ICO conversion factor for the galactic disks and a third of this value for the bridge gas, about 10% of the molecular gas mass is located in the bridge region. The giant HII region close to UGC 12915 is located at the northern edge of the high-surface brightness giant molecular cloud association (GMA), which has the highest velocity dispersion among the bridge GMAs. The bridge GMAs are clearly not virialized because of their high velocity dispersion. Three dynamical models are presented and while no single model reproduces all of the observed features, they are all present in at least one of the models. Most of the bridge gas detected in CO does not form stars. We suggest that turbulent adiabatic compression is responsible for the exceptionally high velocity dispersion of the molecular ISM and the suppression of star formation in the Taffy bridge. In this scenario the turbulent velocity dispersion of the largest eddies and turbulent substructures/clouds increase such that giant molecular clouds are no longer in global virial equilibrium. The increase of the virial parameter leads to a decrease of the star formation efficiency. Most of the low-surface density, CO-emitting gas will disperse without forming stars but some of the high-density gas will probably collapse and form dense star clusters, such as the luminous HII region close to UGC 12915. We suggest that globular clusters and super star clusters formed and still form through the gravitational collapse of gas previously compressed by turbulent adiabatic compression during galaxy interactions.
We present measurements of the star formation efficiency (SFE) in 3D numerical simulations of driven turbulence in supercritical, ideal-MHD, and non-magnetic regimes, characterized by their mean normalized mass-to-flux ratio $mu$, all with 64 Jeans masses and similar rms Mach numbers ($sim 10$). In most cases, the moderately supercritical runs with $mu = 2.8$ have significantly lower SFEs than the non-magnetic cases, being comparable to observational estimates for whole molecular clouds ($lesssim$ 5% over 4 Myr). Also, as the mean field is increased, the number of collapsed objects decreases, and the median mass of the collapsed objects increases. However, the largest collapsed-object masses systematically occur in the weak-field case $mu = 8.8$. The high-density tails of the density histograms in the simulations are depressed as the mean magnetic field strength is increased. This suggests that the smaller numbers and larger masses of the collapsed objects in the magnetic cases may be due to a greater scarcity and lower mean densities (implying larger Jeans masses) of the collapse candidates. In this scenario, the effect of a weak field is to reduce the probability of a core reaching its thermal Jeans mass, even if it is supercritical. We thus suggest that the SFE may be monotonically reduced as the field strength increases from zero to subcritical values, rather than there being a discontinuous transition between the sub- and supercritical regimes, and that a crucial question to address is whether the turbulence in molecular clouds is driven or decaying, with current observational and theoretical evidence favoring (albeit inconclusively) the driven regime.
Supernovae are the most energetic stellar events and influence the interstellar medium by their gasdynamics and energetics. By this, both also affect the star formation positively and negatively. In this paper, we review the complexity of investigations aiming at understanding the interchange between supernova explosions with the star-forming molecular clouds. Commencing from analytical studies the paper advances to numerical models of supernova feedback from superbubble scales to galaxy structure. We also discuss parametrizations of star-formation and supernova-energy transfer efficiencies. Since evolutionary models from the interstellar medium to galaxies are numerous and are applying multiple recipes of these parameters, only a representative selection of studies can be discussed here.
We compare the structure of molecular gas at $40$ pc resolution to the ability of gas to form stars across the disk of the spiral galaxy M51. We break the PAWS survey into $370$ pc and $1.1$ kpc resolution elements, and within each we estimate the molecular gas depletion time ($tau_{rm Dep}^{rm mol}$), the star formation efficiency per free fall time ($epsilon_{rm ff}$), and the mass-weighted cloud-scale (40 pc) properties of the molecular gas: surface density, $Sigma$, line width, $sigma$, and $bequivSigma/sigma^2proptoalpha_{rm vir}^{-1}$, a parameter that traces the boundedness of the gas. We show that the cloud-scale surface density appears to be a reasonable proxy for mean volume density. Applying this, we find a typical star formation efficiency per free-fall time, $epsilon_{ff} left( left< Sigma_{40pc} right> right) sim 0.3{-}0.36%$, lower than adopted in many models and found for local clouds. More, the efficiency per free fall time anti-correlates with both $Sigma$ and $sigma$, in some tension with turbulent star formation models. The best predictor of the rate of star formation per unit gas mass in our analysis is $b equiv Sigma / sigma^2$, tracing the strength of self gravity, with $tau_{rm Dep}^{rm mol} propto b^{-0.9}$. The sense of the correlation is that gas with stronger self-gravity (higher $b$) forms stars at a higher rate (low $tau_{rm Dep}^{rm mol}$). The different regions of the galaxy mostly overlap in $tau_{rm Dep}^{rm mol}$ as a function of $b$, so that low $b$ explains the surprisingly high $tau_{rm Dep}^{rm mol}$ found towards the inner spiral arms found by by Meidt et al. (2013).
We use Atacama Large Millimeter Array CO(3-2) observations in conjunction with optical observations from the Hubble Space Telescope to determine the ratio of stellar to gas mass for regions in the Antennae Galaxies. We adopt the term instantaneous mass ratio IMR(t) = M$_{stars}$/(M$_{gas}$ +M$_{stars}$), that is equivalent to the star formation efficiency for an idealized system at t = 0. We use two complementary approaches to determining the IMR(t) based on 1) the enclosed stellar and molecular mass within circular apertures centered on optically-identified clusters, and 2) a tessellation algorithm that defines regions based on CO emission. We find that only a small number of clusters appear to have IMR(0) = SFE > 0.2, which suggests that only a small fraction of these clusters will remain bound. The results suggest that by ages of $10^{6.7}$ years, some clusters will have lost all of their associated molecular gas, and by $10^{7.5}$ years this is true for the majority of clusters. There appears to be slight dependence of the IMR(t) on the CO surface brightness, which could support the idea that dense molecular environments are more likely to form bound clusters. However, the IMR(t) appears to have a strong dependence on extinction, which likely traces the evolutionary state of clusters.
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