A UNIVERSAL, LOCAL STAR FORMATION LAW IN GALACTIC CLOUDS, NEARBY GALAXIES, HIGH-REDSHIFT DISKS, AND STARBURSTS

By Mark R. Krumholz, A. Dekel, C. McKee. 2011

375 citations recorded by the source corpus.

Star formation laws are rules that relate the rate of star formation in a particular region, either an entire galaxy or some portion of it, to the properties of the gas, or other galactic properties, in that region. While observations of Local Group galaxies show a very simple, local star formation law in which the star formation rate per unit area in each patch of a galaxy scales linearly with the molecular gas surface density in that patch, recent observations of both Milky Way molecular clouds and high-redshift galaxies apparently show a more complicated relationship in which regions of equal molecular gas surface density can form stars at quite different rates. These data have been interpreted as implying either that different star formation laws may apply in different circumstances, that the star formation law is sensitive to large-scale galaxy properties rather than local properties, or that there are high-density thresholds for star formation. Here we collate observations of the relationship between gas and star formation rate from resolved observations of Milky Way molecular clouds, from kpc-scale observations of Local Group galaxies, and from unresolved observations of both disk and starburst galaxies in the local universe and at high redshift. We show that all of these data are in fact consistent with a simple, local, volumetric star formation law. The apparent variations stem from the fact that the observed objects have a wide variety of three-dimensional size scales and degrees of internal clumping, so even at fixed gas column density the regions being observed can have wildly varying volume densities. We provide a simple theoretical framework to remove this projection effect, and we use it to show that all the data, from small solar neighborhood clouds with masses ∼103 M☉ to submillimeter galaxies with masses ∼1011 M☉, fall on a single star formation law in which the star formation rate is simply ∼1% of the molecular gas mass per local free-fall time. In contrast, proposed star formation laws in which the star formation timescale is set by the galactic rotation period are inconsistent with the data from the Milky Way and the Local Group, while those in which the star formation rate is linearly proportional to the gas mass above some density threshold fail both in the Local Group and for starburst galaxies.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/503c72b7b6e10ce893b28f50181c048b32cbc791

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In Astronomy and Astrophysics

1. P. Hopkins
2. L. Hernquist
3. R. Somerville
4. A. Dekel
5. R. Dav'e
6. Mark R. Krumholz
7. E. Quataert
8. T. Naab

Citations in the corpus

• RADIATION-HYDRODYNAMIC SIMULATIONS OF THE FORMATION OF ORION-LIKE STAR CLUSTERS. II. THE INITIAL MASS FUNCTION FROM WINDS, TURBULENCE, AND RADIATION (2012, background): “…by observations even in the absence of the problems rapid star formation creates in the IMF, because observed star formation rates per free-fall time are always a few percent across a very wide range of star-forming environments (Krumholz & Tan 2007; Evans et al. 2009; Krumholz et al. 2012).”
• A sub-parsec resolution simulation of the Milky Way: global structure of the interstellar medium and properties of molecular clouds (2013, background): “Physical interpretations of this threshold are discussed in Krumholz et al. (2012) and Renaud et al. (2012).”
• Inefficient star formation through turbulence, magnetic fields and feedback (2015, background): “The extragalactic data of disc (D) and starburst (SB) galaxies at low and high redshift were compiled in Krumholz et al. (2012a, 2013), and summarized and extended by the Small Magellanic Cloud (SMC) in Federrath (2013b).”
• Feedback-regulated star formation in molecular clouds and galactic discs (2013, background): “We note that the overall scatter in the galff values inferred by Krumholz et al. (2012) for the molecule-rich galaxies considered in this work is comparable to the factor ∼ 100 we find (Figs 5-7), regardless of whether we assume a continuously-varying or bimodal αCO conversion factor (Fig.”
• STAR FORMATION RELATIONS IN NEARBY MOLECULAR CLOUDS (2014, methodology): “3 of Krumholz et al. (2012) effectively mixes our Fig.”
• A Redshift-independent Efficiency Model: Star Formation and Stellar Masses in Dark Matter Halos at z ≳ 4 (2018, background): “Along similar lines, Krumholz et al. (2012) argue that in high-z galaxies, star-forming regions are unable to decouple from the ambient interstellar medium (ISM), the result being that the free-fall times are then set by the large-scale properties of the ISM.”
• The lifecycle of molecular clouds in nearby star-forming disc galaxies (2019, background): “As discussed in Section 5.4, this close agreement with both time-scales is expected when galactic dynamics set the time-scale for cloud evolution (this is referred to as the ‘Toomre regime’ by Krumholz et al. 2012), because the cloud dynamics adjust to the galactic dynamics in this regime.”
• A general model for the CO–H2 conversion factor in galaxies with applications to the star formation law (2011, methodology): “We infer tff from the observable properties of the galaxy (using the approximations of Krumholz et al. 2012), together with either a bimodal XCO or our favoured continuous XCO.”
• THE DEPENDENCE OF QUENCHING UPON THE INNER STRUCTURE OF GALAXIES AT 0.5 ⩽ z < 0.8 IN THE DEEP2/AEGIS SURVEY (2012, result): “Moreover, the predicted star formation efficiency at such low densities is ten times lower than in normal starforming galaxies (Krumholz et al. 2012), which agrees well with the low star-formation efficiencies seen in the Arecibo GASS survey (Schiminovich et al. 2010; Fabello et al. 2011).”
• The big problems in star formation: The star formation rate, stellar clustering, and the initial mass function (2014, methodology): “(2) Krumholz et al. (2007) and Krumholz et al. (2012a) collect a large sample of observations, including both resolved regions of galaxies and entire galaxies, and conclude that all the data are consistent with a universal value ff ∼ 0.01.”
• THE LONG LIVES OF GIANT CLUMPS AND THE BIRTH OF OUTFLOWS IN GAS-RICH GALAXIES AT HIGH REDSHIFT (2013, background): “…relations between the surface densities of gas and star formation rate in a large range of objects (e.g. Elmegreen 2002; Renaud et al. 2012; Krumholz et al. 2012), and when applied to the present simulations matches the gas 8 The metallicity of massive galaxies at redshift 1-2 is high…”
• CHEMICAL CARTOGRAPHY WITH APOGEE: METALLICITY DISTRIBUTION FUNCTIONS AND THE CHEMICAL STRUCTURE OF THE MILKY WAY DISK (2015, background): “…height of the star-forming gas layer decreases with time, in combination with flaring of mono-age populations with radius (e.g., Minchev et al. 2015), as the level of turbulence associated with vigorous star formation decreases (e.g., Bournaud et al. 2009; Krumholz et al. 2012; Bird et al. 2013).”
• Dusty Star Forming Galaxies at High Redshift (2014, background): “Krumholz et al. (2012) developed a model in which the star formation rate operates on some fraction of the small-scale gas free fall time, and can be parametrized by a relation as in Eq.”