RE-EXAMINING LARSON'S SCALING RELATIONSHIPS IN GALACTIC MOLECULAR CLOUDS

By M. Heyer, Coleman M Krawczyk, J. Duval, J. Jackson. 2008

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The properties of Galactic molecular clouds tabulated by Solomon et al. (SRBY) are re-examined using the Boston University-FCRAO Galactic Ring Survey of 13CO J = 1–0 emission. These new data provide a lower opacity tracer of molecular clouds and improved angular and spectral resolution compared with previous surveys of molecular line emission along the Galactic Plane. We calculate giant molecular cloud (GMC) masses within the SRBY cloud boundaries assuming local thermodynamic equilibrium (LTE) conditions throughout the cloud and a constant H2 to 13CO abundance, while accounting for the variation of the 12C/13C with galactocentric radius. The LTE-derived masses are typically five times smaller than the SRBY virial masses. The corresponding median mass surface density of molecular hydrogen for this sample is 42 M☉ pc−2, which is significantly lower than the value derived by SRBY (median 206 M☉ pc−2) that has been widely adopted by most models of cloud evolution and star formation. This discrepancy arises from both the extrapolation by SRBY of velocity dispersion, size, and CO luminosity to the 1 K antenna temperature isophote that likely overestimates the GMC masses and our assumption of constant 13CO abundance over the projected area of each cloud. Owing to the uncertainty of molecular abundances in the envelopes of clouds, the mass surface density of GMCs could be larger than the values derived from our 13CO measurements. From velocity dispersions derived from the 13CO data, we find that the coefficient of the cloud structure functions, v° = σv/R1/2, is not constant, as required to satisfy Larson's scaling relationships, but rather systematically varies with the surface density of the cloud as ∼Σ0.5 as expected for clouds in self-gravitational equilibrium.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/4a867a53bb204a2c390e5bba7358b9261b18e871

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Citations in the corpus

PHYSICAL PROPERTIES OF MOLECULAR CLOUDS FOR THE ENTIRE MILKY WAY DISK (2016, methodology): “The tightest correlation is found to be σv = 0.23 (ΣR)0.43±0.14, in agreement with Heyer et al. (2009).”
Gravity or turbulence? II. Evolving column density PDFs in molecular clouds (2011, background): “…they found that the velocity dispersion should scale as δv2 ≃ 2GΣR (1) where δv is the velocity dispersion of the gas, Σ is the column density of the core, R its size, andG the universal constant of gravity, according to the Heyer et al. (2009) relation recently found for molecular clouds.”
Turbulent molecular clouds (2012, background): “In their analysis of the Galactic Ring survey in 13CO(1–0) emission, Heyer et al. (2009) find that σ/R1/2 ∝ Σ1/2, and that the molecular clouds sampled by this survey are in virial balance.”
On the fraction of star formation occurring in bound stellar clusters (2012, background): “A fiducial value of αvir = 1.3 (McKee & Tan 2003) is adopted here, but it should be noted that values in the range αvir = 0.2–10 are found in observations (Solomon et al. 1987; Heyer et al. 2009) and numerical work (Tasker & Tan 2009; Dobbs et al. 2011b; Hopkins, Quataert & Murray 2012).”
PHYSICAL PROPERTIES AND GALACTIC DISTRIBUTION OF MOLECULAR CLOUDS IDENTIFIED IN THE GALACTIC RING SURVEY (2010, methodology): “In addition, the lack of sampling, resolution, and sensitivity of CO surveys has made the derivation of the physical properties and structure of molecular clouds difficult (Heyer et al. 2009).”
FIRE-2 simulations: physics versus numerics in galaxy formation (2017, background): “…of gas within clouds, versus their projected mass-weighted rms radius), to observations of nearby galaxies ((Bolatto et al. 2008; Fukui et al. 2008; Heyer et al. 2009; Muraoka et al. 2009; Roman-Duval et al. 2010; Colombo et al. 2014; Heyer & Dame 2015; Tosaki et al. 2017); note our definition of…”
THE MAGELLANIC MOPRA ASSESSMENT (MAGMA). I. THE MOLECULAR CLOUD POPULATION OF THE LARGE MAGELLANIC CLOUD (2011, background): “While such a correlation arises trivially when using virial masses to derive Σ, the fact that Heyer et al. (2009) use masses from an LTE analysis of13CO leads them to argue in favor of clouds being gravitationally bound.”
Why are most molecular clouds not gravitationally bound (2011, background): “In a recent observational study, Heyer et al. (2009) published revised estimates of molecular cloud masses, sizes and virial parameters from the previous seminal work by Solomon et al. (1987).”
Star Formation in the Milky Way and Nearby Galaxies (2012, background): “Heyer et al. (2009) reanalyzed cloud properties in the inner Galaxy based on CO and 13CO surveys.”
Gravity or turbulence? Velocity dispersion–size relation (2010, methodology): “From this figure we note that typical local clouds with mean column densities of 1021–1022 cm−2 will necessarily be close to Larson’s relation, as observed in the data (e.g., Larson 1981; Heyer et al. 2009).”
THE DARK MOLECULAR GAS (2010, methodology): “Heyer et al. (2009) emphasized the importance of the dependence of the linewidth on column density and showed that the data on Galactic molecular clouds are consistent with this scaling with αvir = 1.”
The structure of the interstellar medium of star‐forming galaxies (2011, background): “Observations have even found evidence for a systematic shift in the normalization of the size–mass relation (〈〉) towards lower densities in SMC clouds and higher densities in dense regions of massive galaxies (Bolatto et al. 2008; Heyer et al. 2009).”
MOLECULAR GAS AND STAR FORMATION IN NEARBY DISK GALAXIES (2013, methodology): “We scale the conversion factor by this expression, so that: αCO∝ 1 1− fCO−dark (B3) αCO∝ ( exp (−4.0∆AV,DG ĀV (D/G) ))−1 (B4) αCO∝ exp ( 4.0∆AV,DG ĀV (D/G) ) (B5) Adopting the conversion between dust and column density of Bohlin et al. (1978) and a fiducial average molecular cloud surface density ”
The lifecycle of molecular clouds in nearby star-forming disc galaxies (2019, methodology): “This spatial scale is close to the typical sizes of GMCs measured in the Milky Way (Solomon et al. 1987; Heyer et al. 2009; Miville-Deschênes et al. 2017), implying that the galaxy sample is suitable for constraining the GMC lifecycle using our methodology (see Sections 3 and 6).”
The CO-H2 Conversion Factor in Disc Galaxies and Mergers (2011, background): “GMCs inthe Galaxy and local Universe appear to have a nearly constant surface density of 85-100 M⊙ /pc2, obey the size-linewidth relation, and have relatively low kinetic temperatures of 10-20 K (Solomon et al. 1987; Blitz et al. 2007; Heyer et al. 2009).”
THE RELATION BETWEEN GAS AND DUST IN THE TAURUS MOLECULAR CLOUD (2010, background): “The resulting surface density is ∼39M⊙ pc−2 which is very similar to the median value of 42M⊙ pc −2 derived from a large sample of galactic molecular clouds by Heyer et al. (2009).”
Global hierarchical collapse in molecular clouds. Towards a comprehensive scenario (2019, background): “…(e.g., Williams & Blitz 1998) (ii) Clouds appear to be close to equipartition between their kinetic and gravitational energies, except at low column densities, where large kinetic energy excesses are often found (e.g., Larson 1981; Heyer et al. 2009; Kauffmann et al. 2013; Leroy et al. 2015).”