Star Formation from Galaxies to Globules

By Bruce G. Elmegreen. 2001

263 citations recorded by the source corpus.

The origin of the empirical laws of galactic scale star formation is considered in view of the self-similar nature of interstellar gas and the observation that most local clusters are triggered by specific high-pressure events. The empirical laws suggest that galactic scale gravity is involved in the first stages of star formation, but they do not identify the actual triggering mechanisms for clusters in the final stages. Many triggering processes satisfy the empirical laws, including turbulence compression and expanding shell collapse. The self-similar nature of the gas and associated young stars suggests that turbulence is more directly involved, but the energy source for this turbulence is not clear, and the small-scale morphology of gas around most embedded clusters does not look like a random turbulent flow. Most clusters appear to be triggered by other nearby stars. Such a prominent local influence makes it difficult to understand the universality of the Kennicutt and Schmidt laws on galactic scales. A unified view of multiscale star formation avoids most of these problems. The Toomre and Kennicutt surface density thresholds, along with the large-scale gas and star formation morphology, imply that ambient self-gravity produces spiral arms and giant cloud complexes and at the same time drives much of the turbulence that leads to self-similar structures. Localized energy input from existing clusters and field supernovae drives turbulence and cloud formation too, while triggering clusters directly in preexisting clouds. The hierarchical structure in the gas made by turbulence ensures that the triggering time scales with size, thereby giving the Schmidt law over a wide range of scales and the size-duration correlation for young star fields. Reanalysis of the Schmidt law from a local point of view suggests that the efficiency of star formation is determined by the fraction of the gas above a critical density of around 105 m(H2) cm-3. Such high densities probably result from turbulence compression in a self-gravitating gas, in which case their mass fraction can be estimated from the density distribution function that results from turbulence. For Wada & Norman's lognormal function that arises in whole-galaxy simulations, the theoretically predicted mass fraction of star-forming material is the same as that observed directly from the galactic Schmidt law and is ~10-4. The unified view explains how independent star formation processes can combine into the empirical laws while preserving the fractal nature of interstellar gas and the pressurized, wind-swept appearance of most small-scale clouds. Likely variations in the relative roles of these processes from region to region should not affect the large-scale average star formation rate. Self-regulation by spiral instabilities and star formation ensures that most regions are in a marginally stable state in which turbulence limits the mass available for star formation and the overall rate is independent of the nature of the energy sources. In this sense, star formation is saturated to its largest possible value given the fractal nature of the interstellar medium.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/a20fda3e7ad4076d8972f4c51eb5a28c86b2a86a

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Citations in the corpus

A UNIVERSAL, LOCAL STAR FORMATION LAW IN GALACTIC CLOUDS, NEARBY GALAXIES, HIGH-REDSHIFT DISKS, AND STARBURSTS (2011, unknown): “One simple approach is to hypothesize that the relevant density is simply the galactic midplane density, and to estimate this by assuming that all galaxies have roughly the same scale height (e.g. Elmegreen 2002).”
On the relation between the Schmidt and Kennicutt-Schmidt star formation laws and its implications for numerical simulations (2007, background): “Many theories have been proposed to explain the observed KS law (e.g. Silk 1997; Tan 2000; Elmegreen 2002; Krumholz & McKee 2005; Tutukov 2006).”
Star-formation laws in luminous infrared galaxies. New observational constraints on models (2011, background): “Alternatively, other forms of the star formation laws adduce the importance of including explicitlyglobal dynamical time scales tdyn (Elmegreen 1997; Silk 1997; Kennicutt 1998; Elmegreen 2002; Boissier et al. 2003; Tasker & Tan 2009; Tan 2010; Genzel et al. 2010).”
The Formation of Stellar Clusters in Turbulent Molecular Clouds: Effects of the Equation of State (2002, background): “A theory of galactic-scale star formation partially based on the universality of log-normal density PDFs has been proposed by Elmegreen (2002).”
Radial Variation of Attenuation and Star Formation in the Largest Late-Type Disks Observed with GALEX (2006, background): “An excellent review of the empirical situation is given in Kennicutt (1998b), while Elmegreen (2002) consider the possible physical origin of the empirical laws.”
Molecular Hydrogen and Global Star Formation Relations in Galaxies (2007, methodology): “Although several models were proposed to explain such a correlation (see, e.g., Kennicutt 1998; Elmegreen 2002, for reviews), we show in §3.4 and the second Appendix that the star formation correlation with Ω can be understood as a fortuitous correlation of Ω with gas density ofΩ ∝ ραgas, whereα ≈…”
Star Formation in NGC 5194 (M51a). II. The Spatially Resolved Star Formation Law (2007, background): “…mechanisms, including large-scale gravitational instabilities (e.g., Quirk & Tinsley 1973; Zasov & Simakov 1988; Kennicutt 1989; Hunter et al. 1998; Elmegreen 2002), or molecular or cold gas phase formation thresholds (e.g., Elmegreen & Parravano 1994; Schaye 2004; Blitz & Rosolowsky 2004).”
The Molecular Gas Distribution and Schmidt Law in M33 (2003, background): “Star formation within galaxies is commonly described by two relationships, which are oversimplifications of complex interstellar processes taking place over a large range of spatial scales (Elmegreen 2002).”
Theory of Star Formation (2007, background): “Elmegreen (2002, 2003) suggested that the star formation rate per unit volume is ρ̇∗ ≃ ǫcorefcore(Gρcore)1/2ρ, where fcore is the fraction of gas in dense cores; the value of this was set by comparison with observation.”
Why are most molecular clouds not gravitationally bound (2011, background): “Alternatively, in light of recent observational evidence, one can assume that clouds are typically short-lived entities (Hartmann et al. 2001; Elmegreen 2002, 2007; Ballesteros-Paredes et al. 2007) and posit mechanisms which prevent most of the gas forming stars, such as stellar feedback and…”
Comparing the statistics of interstellar turbulence in simulations and observations - Solenoidal versus compressive turbulence forcing (2009, background): “…to explain the stellar initial mass function (Padoan & Nordlund 2002; Hennebelle & Chabrier 2008), the star formation rate (Krumholz & McKee 2005), the star formation efficiency (Elmegreen 2008), and the Kennicutt-Schmidt relation on galactic scales (Elmegreen 2002; Kravtsov 2003; Tassis 2007).”
On the Origin of the Global Schmidt Law of Star Formation (2003, methodology): “A wide variety of possible models with such scaling have been proposed in the last three decades (see Elmegreen 2002, for a review).”
Formation of Globular Clusters in Hierarchical Cosmology (2003, background): “In particular, mergers stir and compress the interstellar gas creating the high-pressure environments in which dense clouds and massive stellar clusters can form (Elmegreen 2002).”
On the fraction of star formation occurring in bound stellar clusters (2012, methodology): “2 for the two different star formation laws (Elmegreen 2002; Krumholz & McKee 2005) that are adopted in this paper.”
On the frequency, intensity, and duration of starburst episodes triggered by galaxy interactions and mergers (2008, methodology): “If the Schmidt law results from the dynamical timescale of gas clouds 1/ √ Gρ (Elmegreen 2002), it should not be scale free, and the density should be computed when clouds begin to form on rather large scale, not at smaller scales when they have begun to collapse.”
The KMOS Redshift One Spectroscopic Survey (KROSS):dynamical properties, gas and dark matter fractions of typical z~1 star-forming galaxies (2016, background): “It is such gravitational instabilities that are thought to induce the star formation in disc galaxies by causing the gas to clump and condense (Elmegreen 2002; Li, Mac Low & Klessen 2005).”
MORPHOLOGICAL QUENCHING OF STAR FORMATION: MAKING EARLY-TYPE GALAXIES RED (2009, background): “Gravitational instability is a necessary condition for efficient star formation in a galactic disk (e.g., Elmegreen 2002; Li et al. 2005; Kawata et al. 2007) The instability of a single-component, rotating disk at radius r is characterized by the Toomre Q parameter (Toomre 1964): Q = κσ ǫGΣ .”
An uncertainty principle for star formation - I. Why galactic star formation relations break down below a certain spatial scale (2014, background): “Galactic star formation (SF) relations (e.g. Silk 1997; Kennicutt 1998; Elmegreen 2002; Bigiel et al. 2011) break down below a certain spatial scale (Schruba et al. 2010; Onodera et al. 2010; Liu et al. 2011).”
WHAT IS DRIVING THE H i VELOCITY DISPERSION? (2009, background): “The physical explanation might be that supercritical density fluctuations are often dispersed on timescales shorter than the free-fall time, arresting the collapse (Klessen et al. 2000; Elmegreen 2002; Joung & Mac Low 2006).”