The Resolved Properties of Extragalactic Giant Molecular Clouds

By A. Bolatto, A. Leroy, Erik Rosolowsky, F. Walter, L. Blitz. Proceedings of the International Astronomical Union, 2008

407 citations recorded by the source corpus.

We use high spatial resolution observations of CO to systematically measure the resolved size-line width, luminosity-line width, luminosity-size, and mass-luminosity relations of GMCs in a variety of extragalactic systems. Although the data are heterogeneous, we analyze them in a consistent manner to remove the biases introduced by limited sensitivity and resolution, thus obtaining reliable sizes, velocity dispersions, and luminosities. We compare the results obtained in dwarf galaxies with those from the Local Group spiral galaxies. We find that extragalactic GMC properties measured across a wide range of environments are very much compatible with those in the Galaxy. The property that shows the largest variability is their resolved brightness temperature, although even that is similar to the average Galactic value in most sources. We use these results to investigate metallicity trends in the cloud average column density and virial CO-to-H2 factor. We find that these measurements do not accord with simple predictions from photoionization-regulated star formation theory, although this could be due to the fact that we do not sample small enough spatial scales or the full gravitational potential of the molecular cloud. We also find that the virial CO-to-H2 conversion factor in CO-bright GMCs is very similar to Galactic and that the excursions do not show a measurable metallicity trend. We contrast these results with estimates of molecular mass based on far-infrared measurements obtained for the Small Magellanic Cloud, which systematically yield larger masses, and interpret this discrepancy as arising from large H2 envelopes that surround the CO-bright cores. We conclude that GMCs identified on the basis of their CO emission are a unique class of objects that exhibit a remarkably uniform set of properties from galaxy to galaxy.

Subfields: Astronomy and Astrophysics, Atmospheric Science

View Paper: https://www.semanticscholar.org/paper/3ad4218773dddb16089accd89bc2be2ce1d3f1b1

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Citations in the corpus

COLD GASS, an IRAM legacy survey of molecular gas in massive galaxies - II. The non-universality of the molecular gas depletion time-scale (2011, background): “Krumholz, Leroy & McKee (2011) suggest that COinferred molecular gas measurements may begin to miss an increasing fraction of the total H2 at metallicities as high as half-solar (see also Bolatto et al. 2008; Leroy et al. 2011).”
MAXIMALLY STAR-FORMING GALACTIC DISKS. I. STARBURST REGULATION VIA FEEDBACK-DRIVEN TURBULENCE (2011, methodology): “The value Σ ∼ 100 M⊙ pc−2 that defines the upper limit for “main disks” is characteristic of the surface density ΣGMC of individual giant molecular clouds (GMCs), as observed in the Milky Way and Local Group (Blitz et al. 2007; Sheth et al. 2008; Bolatto et al. 2008).”
THE CO-TO-H2 CONVERSION FACTOR AND DUST-TO-GAS RATIO ON KILOPARSEC SCALES IN NEARBY GALAXIES (2012, methodology): “Bolatto et al. (2008) derived GMC properties and virial masses for NGC 2976 and 3077.”
A UNIVERSAL, LOCAL STAR FORMATION LAW IN GALACTIC CLOUDS, NEARBY GALAXIES, HIGH-REDSHIFT DISKS, AND STARBURSTS (2011, unknown): “These survive for ∼ 30 Myr (∼ 5−10 free-fall times; Fukui et al. 2009), and their properties are observed to be independent of the Galactic environment (Bolatto et al. 2008).”
Molecular Clouds in the Milky Way (2015, background): “Thus, ALMA can extend many of the analyses discussed in this review to molecular clouds residing within nearby galaxies (Bolatto et al. 2008, Colombo et al. 2014, Leroy et al. 2015).”
THE NATURE OF STARBURSTS. I. THE STAR FORMATION HISTORIES OF EIGHTEEN NEARBY STARBURST DWARF GALAXIES (2010, background): “…ISM is rarely detectable in dwarfs (e.g., Israel et al. 1995; Taylor et al. 1998; Barone et al. 2000), and thus is unlikely to contribute significantly to the gas supply, unless the uncertain conversion factor from CO to H2 is extremely high (e.g., Wilson 1995; Israel 1997; Bolatto et al. 2008).”
THE STATE OF THE GAS AND THE RELATION BETWEEN GAS AND STAR FORMATION AT LOW METALLICITY: THE SMALL MAGELLANIC CLOUD (2011, background): “They differ, however, from estimates based on small-scale virial masses toward the CO-bright peaks, which tend to obtain values of αCO closer to Galactic (Israel et al. 2003; Blitz et al. 2007; Bolatto et al. 2008; Müller et al. 2010).”
The SILCC (SImulating the LifeCycle of molecular Clouds) project - II. Dynamical evolution of the supernova-driven ISM and the launching of outflows (2015, background): “…dispersion in molecular hydrogen (directly determined from H2 in the simulations) agrees well with observed (in CO) molecular gas complexes or giant molecular clouds (GMCs) in M31 (σ = 6.5 ± 1.2 km s−1, Sheth et al. 2008) and GMCs in other star forming galaxies (1−10 km s−1, Bolatto et al. 2008).”
Cold gas properties of the Herschel Reference Survey - II. Molecular and total gas scaling relations (2014, background): “…of the interstellar radiation field, is still far from being settled both observationally and theoretically (Boselli et al. 200, Bell et al. 2007, Bolatto et al. 2008, Liszt et al. 2010, Leroy et al. 2011, Shetty et al. 2011a,b, Narayanan et al. 2012, Kennicutt & Evans 2012, Bolatto et al. 2013,…”
REGULARITY UNDERLYING COMPLEXITY: A REDSHIFT-INDEPENDENT DESCRIPTION OF THE CONTINUOUS VARIATION OF GALAXY-SCALE MOLECULAR GAS PROPERTIES IN THE MASS-STAR FORMATION RATE PLANE (2013, unknown): “…molecular gas masses for high-redshift galaxies: αCO =4.4M⊙ (K km/s pc 2)−1 (the conversion factor that is found to apply to GMCs in the Milky Way; e.g. Bolatto et al. 2008, Abdo et al. 2010) for normal galaxies and 0.8M⊙ (K km/s pc 2)−1, a representative average for local starbursting ULIRGs…”
The CO-to-H2 Conversion Factor (2013, methodology): “In that sense, the uniformity in XCO derived from virial masses probably reflects fairly uniform conditions in CO-bright regions of molecular clouds (Bolatto et al. 2008).”
STAR FORMATION IN DISK GALAXIES. I. FORMATION AND EVOLUTION OF GIANT MOLECULAR CLOUDS VIA GRAVITATIONAL INSTABILITY AND CLOUD COLLISIONS (2008, background): “…versus size relation to observed GMCs, such as those observed in M33 by Rosolowsky et al. (2003), in the Milky Way by Soloman et al. (1987), or the average of a number of extragalactic systems, σc = σpc(R/pc) ασ with ασ = 0.60 ± 0.10 and σpc = 0.44 ± 0.15 km s−1, derived by Bolatto et al. (2008).”
The CO-H2 Conversion Factor in Disc Galaxies and Mergers (2011, background): “…well (Blitz et al. 2007; Wolfire et al. 2010), though there may be some variations associated with metallicity (Wilson1995; Arimoto et al. 1996; Boselli et al. 2002; Rosolowsky et al. 2003; Israel 2005; Bell et al. 2006; Leroy et al. 2006; Bell et al. 2007; Bolatto et al. 2008; Leroy et al. 2011).”
On the fine structure of the Cepheid metallicity gradient in the Galactic thin disk (2014, background): “3 in Bolatto et al. 2008 and Table 1 in Murray 2011) and to the typical size of giant star complexes Article number, page 7 of 34 and superassociations (Elmegreen & Elmegreen 1983; Efremov 1995).”
The DiskMass Survey VII. The distribution of luminous and dark matter in spiral galaxies (2013, methodology): “Distribution of luminous and dark matter in spiral galaxies Galactic measurement of XCO from Dame et al. (2001) with the measurements for M 31 and M 33 from Bolatto et al. (2008).”
On the relationship between molecular hydrogen and carbon monoxide abundances in molecular clouds (2010, methodology): “…assume that extragalactic GMCs are in virial equilibrium and use a virial analysis to determine the cloud mass generally find values for XCO that are similar to those obtained in the Milky Way, with at most a weak metallicity dependence (Wilson 1995; Rosolowsky et al. 2003; Bolatto et al. 2008).”
A study of the gas–star formation relation over cosmic time (2010, background): “Because of photo-dissociation of the molecular gas, the situation in low-metallicity (<50 per cent solar) dwarf systems is more uncertain, although even there a Galactic conversion appears to apply in the dense star-forming clumps themselves (Bolatto et al. 2008).”
THE MAGELLANIC MOPRA ASSESSMENT (MAGMA). I. THE MOLECULAR CLOUD POPULATION OF THE LARGE MAGELLANIC CLOUD (2011, methodology): “5 was derived by Solomon et al. (1987) for the inner Galaxy, and a similar relation with slope of 0.60± 0.10 was derived by Bolatto et al. (2008) for extragalactic clouds.”