Slow Star Formation in Dense Gas: Evidence and Implications

By Mark R. Krumholz, Jonathan C. Tan. 2006

517 citations recorded by the source corpus.

It has been known for more than 30 years that star formation in giant molecular clouds (GMCs) is slow, in the sense that only ~1% of the gas forms stars every free-fall time. This result is entirely independent of any particular model of molecular cloud lifetime or evolution. Here we survey observational data on higher density objects in the interstellar medium, including infrared dark clouds and dense molecular clumps, to determine whether these objects form stars slowly like GMCs, or rapidly, converting a significant fraction of their mass into stars in one free-fall time. We find no evidence for a transition from slow to rapid star formation in structures covering 3 orders of magnitude in density. This has important implications for models of star formation, since competing models make differing predictions for the characteristic density at which star formation should transition from slow to rapid. The data are inconsistent with models that predict that star clusters form rapidly and in free-fall collapse. Magnetic- and turbulence-regulated star formation models can reproduce the observations qualitatively, and the turbulence-regulated star formation model of Krumholz & McKee quantitatively reproduces the infrared-HCN luminosity correlation recently reported by Gao & Solomon Slow star formation also implies that the process of star cluster formation cannot be one of global collapse, but must instead proceed over many free-fall times. This suggests that turbulence in star-forming clumps must be driven, and that the competitive accretion mechanism does not operate in typical cluster-forming molecular clumps.

Subfields: Astronomy and Astrophysics, Atomic and Molecular Physics, and Optics, Organic Chemistry

View Paper: https://www.semanticscholar.org/paper/ff48801b1e5e6e0246954e6fe535013375855cba

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Citations in the corpus

Stellar Feedback in Galaxies and the Origin of Galaxy-scale Winds (2011, background): “…densities, and star formation occurs in dense regions above a threshold n > 1000 cm−3, with a rate ρ̇∗ = ρ/tff where tff is the free-fall time and = 1.5 per cent is an efficiency taken from observations of star-forming regions with the same densities (Krumholz & Tan 2007, and references therein).”
ON THE INTERPLAY BETWEEN STAR FORMATION AND FEEDBACK IN GALAXY FORMATION SIMULATIONS (2014, methodology): “Following Krumholz & Tan (2007), we can write this time scale as tSF = tff,SF/ ff,SF, where tff,SF =√ 3π/32Gρg is the local free-fall time of the star forming gas and ff,SF is the local star formation efficiency per free-fall time.”
THE DISRUPTION OF GIANT MOLECULAR CLOUDS BY RADIATION PRESSURE & THE EFFICIENCY OF STAR FORMATION IN GALAXIES (2009, background): “Star formation is similarly slow on smaller scales within galaxies (Kennicutt et al. 2007; Bigiel et al. 2008; Leroy et al. 2008; Krumholz & Tan 2007).”
The dust content of galaxies from z = 0 to z = 9 (2016, background): “Observations constrain this efficiency to ∼1% (Krumholz & Tan 2007; Krumholz, Dekel & McKee 2012; Krumholz 2014).”
IONIZING PHOTON ESCAPE FRACTIONS FROM HIGH-REDSHIFT DWARF GALAXIES (2008, background): “Gnedin (2000) suggested that, based on Local Group dwarf galaxies, the star formation efficiency at high redshift is ∼ 4%, consistent with theoretical works (e.g. Krumholz & McKee 2005; Krumholz & Tan 2007).”
LOW CO LUMINOSITIES IN DWARF GALAXIES (2012, background): “…(Blitz et al. 2007; Bolatto et al. 2008; Fukui & Kawamura 2010), the universality of SFE per free-fall time in clouds of different mass and density (Krumholz & Tan 2007), and evidence in favor of a universal initial stellar mass function (Bastian et al. 2010); and (b) the remarkably constant…”
THREE-DIMENSIONAL HYDRODYNAMIC SIMULATIONS OF MULTIPHASE GALACTIC DISKS WITH STAR FORMATION FEEDBACK. I. REGULATION OF STAR FORMATION RATES (2013, methodology): “We take a fiducial value ff = 0.01, which is consistent with theory and observations of dense ISM gas (Krumholz & McKee 2005; Krumholz & Tan 2007; Krumholz et al. 2012).”
A general model for the CO–H2 conversion factor in galaxies with applications to the star formation law (2011, background): “Both observations and theory have suggested that the star formation efficiency per free-fall time ff in molecular gas is approximately constant (Krumholz & McKee 2005; Krumholz & Tan 2007; Evans et al. 2009; Ostriker & Shetty 2011; Krumholz, Dekel & McKee 2012).”
ENVIRONMENTAL DEPENDENCE OF THE KENNICUTT–SCHMIDT RELATION IN GALAXIES (2010, methodology): “We follows the definition of Krumholz & Tan (2007) for the gas free-fall time, τff = √ 3π 32Gρ (here ρ is the total mass density, including helium), and τmax is the free-fall time in the gas with nSF = 50 cm−3.”
On the Rapid Collapse and Evolution of Molecular Clouds (2007, methodology): “All of the points in Figure 5 of Krumholz & Tan (2007) other than HCN were for individual regions, as was the Wu et al. (2005) contribution to the HCN point.”
MODELING MOLECULAR HYDROGEN AND STAR FORMATION IN COSMOLOGICAL SIMULATIONS (2008, methodology): “We adopt ǫff = 0.01 (Krumholz & Tan 2007b) in all of the calculations presented below; we also use a molecular fraction threshold for computational efficiency: star formation is allowed only in cells which have a molecular fraction higher than fH2 = 0.1.”
Galaxy formation theory (2010, background): “Recently, new observations have suggested a relatively simple odel for star formation (Krumholz and Tan 2007).”
The formation of disc galaxies in a ΛCDM universe (2010, background): “These values are in agreement with GMC estimates from Krumholz & Tan (2007).”
THE SPATIALLY RESOLVED STAR FORMATION LAW FROM INTEGRAL FIELD SPECTROSCOPY: VIRUS-P OBSERVATIONS OF NGC 5194 (2009, unknown): “…and GMCs in the Milky Way to the molecular ISM in large scales in normal spiral galaxies and starburst, and is consistent with models in which star-formation is regulated by supersonic turbulence in GMCs, induced by feedback from star-formation itself (Evans et al. 2009; Krumholz & Tan 2007).”
THE STAR FORMATION RATE AND GAS SURFACE DENSITY RELATION IN THE MILKY WAY: IMPLICATIONS FOR EXTRAGALACTIC STUDIES (2010, background): “This is possible because even in dense gas, the star formation rate per free-fall time is less than unity (Zuckerman & Evans 1974; Krumholz & Tan 2007).”
Self-regulated growth of supermassive black holes by a dual jet-heating active galactic nucleus feedback mechanism: methods, tests and implications for cosmological simulations (2011, result): “In these simulations, we set the efficiency of star formation to ∗ = 0.02 in good agreement with observational surface density relationships of galaxies (Kennicutt 1998), and local giant molecular clouds (Krumholz & Tan 2007).”
A sub-parsec resolution simulation of the Milky Way: global structure of the interstellar medium and properties of molecular clouds (2013, methodology): “We have adopted ǫ = 3%, in agreement with Krumholz & Tan (2007) who suggested a universal value of a few percent, and measured a global SFR of ∼ 1 – 5 M⊙ yr −1, on average.”
Molecular Hydrogen and Global Star Formation Relations in Galaxies (2007, methodology): “To these ends, we develop a model for the ISM and star formation whose key premise is that star formation on the scales of molecular clouds (∼ 10 pc) is a function of molecular hydrogen density with a universal SFE per free-fall time (e.g., Krumholz & Tan 2007).”