PHYSICAL PROPERTIES AND GALACTIC DISTRIBUTION OF MOLECULAR CLOUDS IDENTIFIED IN THE GALACTIC RING SURVEY

By J. Roman-Duval, J. Jackson, M. Heyer, J. Rathborne, R. Simon. 2010

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We derive the physical properties of 580 molecular clouds based on their 12CO and 13CO line emission detected in the University of Massachusetts-Stony Brook (UMSB) and Galactic Ring surveys. We provide a range of values of the physical properties of molecular clouds, and find a power-law correlation between their radii and masses, suggesting that the fractal dimension of the interstellar medium is around 2.36. This relation, M = (228 ± 18) R2.36 ± 0.04, allows us to derive masses for an additional 170 Galactic Ring Survey (GRS) molecular clouds not covered by the UMSB survey. We derive the Galactic surface mass density of molecular gas and examine its spatial variations throughout the Galaxy. We find that the azimuthally averaged Galactic surface density of molecular gas peaks between Galactocentric radii of 4 and 5 kpc. Although the Perseus arm is not detected in molecular gas, the Galactic surface density of molecular gas is enhanced along the positions of the Scutum-Crux and Sagittarius arms. This may indicate that molecular clouds form in spiral arms and are disrupted in the inter-arm space. Finally, we find that the CO excitation temperature of molecular clouds decreases away from the Galactic center, suggesting a possible decline in the star formation rate with Galactocentric radius. There is a marginally significant enhancement in the CO excitation temperature of molecular clouds at a Galactocentric radius of about 6 kpc, which in the longitude range of the GRS corresponds to the Sagittarius arm. This temperature increase could be associated with massive star formation in the Sagittarius spiral arm.

Subfields: Astronomy and Astrophysics, Instrumentation, Organic Chemistry

View Paper: https://www.semanticscholar.org/paper/766077b0620070abc1a9c737a3fc89b8ec3ee1cf

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Citations in the corpus

The CO-to-H2 Conversion Factor (2013, methodology): “…ΣGMC ≈ 40 M⊙ pc−2 over the same cloud areas but concluding that it is likely an underestimate of at least a factor of 2 due to non-LTE and optical depth effects (ΣGMC ≈ 144 M⊙ pc−2 using a more restrictive 13CO contour instead of the original 12CO to define surface area, Roman-Duval et al. 2010).”
MOLECULAR GAS AND STAR FORMATION IN NEARBY DISK GALAXIES (2013, background): “Much previous work places typical surface densities of Milky Way GMCs at ∼ 150 M⊙ pc −2 (Solomon et al. 1987; Roman-Duval et al. 2010).”
Star Formation in the Milky Way and Nearby Galaxies (2012, methodology): “In a study on the inner Galaxy, Roman-Duval et al. (2010) concluded that 70% of molecular clouds (both in mass and number) were bound.”
The big problems in star formation: The star formation rate, stellar clustering, and the initial mass function (2014, background): “…see the following section), but slightly steeper than that describing the mass function of individual molecular clouds in the molecule-rich parts of galactic disks (roughly 1.5 to 2.0; (Solomon et al. 1987; Heyer et al. 2001; Rosolowsky & Blitz 2005; Roman-Duval et al. 2010; Gratier et al. 2012)).”
LOW VIRIAL PARAMETERS IN MOLECULAR CLOUDS: IMPLICATIONS FOR HIGH-MASS STAR FORMATION AND MAGNETIC FIELDS (2013, methodology): “1, where the results do significantly differ between Heyer et al. (2009) and Roman-Duval et al. (2010), although both studies build on the same set of observations.”
Molecular Clouds in the Milky Way (2015, methodology): “…case in the catalog by Heyer et al. (2001) but is also evident when comparing the most massive clouds in the Galaxy identified by Dame et al. (1986) with underlying substructure for these fields found with higher angular resolution (Solomon et al. 1987, Issa et al. 1990, Roman-Duval et al. 2010).”
PHYSICAL PROPERTIES OF MOLECULAR CLOUDS FOR THE ENTIRE MILKY WAY DISK (2016, background): “Indeed, higher-resolution observations tend to find higher values of nH2 (Roman-Duval et al. 2010).”
Modeling UV Radiation Feedback from Massive Stars. II. Dispersal of Star-forming Giant Molecular Clouds by Photoionization and Radiation Pressure (2018, background): “In this work, we have considered only unmagnetized, marginally bound clouds with αvir,0 = 2, while observed GMCs may have a range of virial values (e.g., Heyer et al. 2009; Roman-Duval et al. 2010; Miville-Deschênes et al. 2017).”
THE STAR FORMATION RATE OF TURBULENT MAGNETIZED CLOUDS: COMPARING THEORY, SIMULATIONS, AND OBSERVATIONS (2012, background): “However, even though most real clouds may roughly follow such an average scaling, the scatter around that average is typically about an order of magnitude or more in terms of mass, density, and velocity dispersion for a given cloud size (e.g., Heyer et al. 2009; Roman-Duval et al. 2010).”
FIRE-2 simulations: physics versus numerics in galaxy formation (2017, background): “…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 Rcloud is equivalent to their σr).”
A UNIVERSAL, LOCAL STAR FORMATION LAW IN GALACTIC CLOUDS, NEARBY GALAXIES, HIGH-REDSHIFT DISKS, AND STARBURSTS (2011, unknown): “We adopt a fiducial value ΣGMC = 85 M⊙ pc −2, the mean found by Bolatto et al. (2008).8 There is a significant scatter about this value from one cloud to another (e.g. Roman-Duval et al. 2010), as is expected from theoretical models (e.g. Goldbaum et al. 2011).”
A Herschel [C ii] Galactic plane survey - I. The global distribution of ISM gas components (2013, background): “It has been observed in large-scale 12CO and 13CO surveys, however, that the spatial extent of the 13CO emission is often noticeably smaller than, and is contained within, the observed extent of the 12CO emission (Goldsmith et al. 2008; Roman-Duval et al. 2010).”
ATLASGAL — towards a complete sample of massive star forming clumps ⋆ (2014, background): “The slope of the clump-mass function is consistent with the upper end of the range found for Galactic samples of giant molecular clouds (GMCs), which are generally found to be between −1.5 and −1.8 (e.g. Solomon et al. 1987; Heyer, Carpenter & Snell 2001; Roman-Duval et al. 2010).”
On the universality of supersonic turbulence (2013, background): “Although this is clearly in the supersonic regime, typical molecular clouds in the Milky Way have Mach numbers of about 5–20 and sizes in the range 1–50 pc (e.g. Roman-Duval et al. 2010).”