Physical Properties and Galactic Distribution of Molecular Clouds Identified in the Galactic Ring Survey

Roman-Duval, Julia; Jackson, James M.; Heyer, M. H.; Rathborne, Jill; Simon, R.

doi:10.1088/0004-637x/723/1/492

Cited by 400 publications

(545 citation statements)

References 72 publications

(146 reference statements)

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“…This arm is prominent in CO (3-2) and therefore has abundant molecular gas content at these longitudes . It is also a strong feature in the RMS source distribution (Urquhart et al 2014a), gas temperature (Roman-Duval et al 2010), and the ratio of infrared luminosity to clump mass (LIR/M clump Moore et al 2012;Eden et al 2015) suggestive of enhancement in the SFE, albeit on the kiloparsec scales probed by earlier surveys. This, however, may be a consequence of local variations (e.g.…”

Section: What Does the Star-forming Fraction (Sff) Mean?mentioning

confidence: 93%

The prevalence of star formation as a function of Galactocentric radius

Ragan

Moore

Eden

et al. 2016

Mon. Not. R. Astron. Soc.

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We present large-scale trends in the distribution of star-forming objects revealed by the Hi-GAL survey. As a simple metric probing the prevalence of star formation in Hi-GAL sources, we define the fraction of the total number of Hi-GAL sources with a 70 µm counterpart as the "star-forming fraction" or SFF. The mean SFF in the inner galactic disc (3.1 kpc < R GC < 8.6 kpc) is 25%. Despite an apparent pile-up of source numbers at radii associated with spiral arms, the SFF shows no significant deviations at these radii, indicating that the arms do not affect the star-forming productivity of dense clumps either via physical triggering processes or through the statistical effects of larger source samples associated with the arms. Within this range of Galactocentric radii, we find that the SFF declines with R GC at a rate of −0.026±0.002 per kiloparsec, despite the dense gas mass fraction having been observed to be constant in the inner Galaxy. This suggests that the SFF may be weakly dependent on one or more largescale physical properties of the Galaxy, such as metallicity, radiation field, pressure or shear, such that the dense sub-structures of molecular clouds acquire some internal properties inherited from their environment.

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Section: What Does the Star-forming Fraction (Sff) Mean?mentioning

confidence: 93%

The prevalence of star formation as a function of Galactocentric radius

Ragan

Moore

Eden

et al. 2016

Mon. Not. R. Astron. Soc.

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“…Note also that the GMCs are somewhat confined by the pressure of the ambient, uniform density medium. Observed GMCs appear to have smaller virial parameters, ∼1 (e.g., Roman-Duval et al 2010;Kauffmann et al 2013), especially when considering their positionvelocity connected, 13 CO-emitting structures (Hernandez & Tan 2015). Our choice of initializing with a larger kinetic energy content is motivated by the desire to not have rapid global collapse of the clouds within the first few megayears, i.e., the timescale of the collision.…”

Section: Initial Conditionsmentioning

confidence: 99%

GMC Collisions as Triggers of Star Formation. II. 3D Turbulent, Magnetized Simulations

Tan

Nakamura

et al. 2017

ApJ

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We investigate giant molecular cloud collisions and their ability to induce gravitational instability and thus star formation. This mechanism may be a major driver of star formation activity in galactic disks. We carry out a series of 3D, magnetohydrodynamics (MHD), adaptive mesh refinement simulations to study how cloud collisions trigger formation of dense filaments and clumps. Heating and cooling functions are implemented based on photodissociation region models that span the atomic-to-molecular transition and can return detailed diagnostic information. The clouds are initialized with supersonic turbulence and a range of magnetic field strengths and orientations. Collisions at various velocities and impact parameters are investigated. Comparing and contrasting colliding and non-colliding cases, we characterize morphologies of dense gas, magnetic field structure, cloud kinematic signatures, and cloud dynamics. We present key observational diagnostics of cloud collisions, especially: relative orientations between magnetic fields and density structures, like filaments; 13 CO( J=2-1), 13 CO( J=3-2), and 12 CO( J=8-7) integrated intensity maps and spectra; and cloud virial parameters. We compare these results to observed Galactic clouds.

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“…Roman-Duval et al (2010) found M ∝ r 2.26 for molecular clouds having sizes in the range 0.7−30 pc. Heithausen et al (1998) found M ∝ r 2.31 for scales ranging from 0.01 to 1 pc.…”

Section: Massmentioning

confidence: 97%

“…In other words, the mass does not scale with L 3 (see Fig. 8), therefore the density estimated over large physical scales is an underestimate of the density at small scales, explaining why the average density in GMCs is found to be smaller than the critical density of CO. For instance, from the sample of molecular clouds of Roman-Duval et al (2010), one obtains that n H2 = 800L −0.64 . For the typical size of a structure in Draco (L = 0.15 pc, see Fig.…”

Section: Gas Densitymentioning

confidence: 99%

Structure formation in a colliding flow: The Herschel view of the Draco nebula

Miville-Deschênes

Salomé

Martín

et al. 2017

A&A

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Context. The Draco nebula is a high Galactic latitude interstellar cloud observed at velocities corresponding to the intermediate velocity cloud regime. This nebula shows unusually strong CO emission and remarkably high-contrast small-scale structures for such a diffuse high Galactic latitude cloud. The 21 cm emission of the Draco nebula reveals that it is likely to have been formed by the collision of a cloud entering the disk of the Milky Way. Such physical conditions are ideal to study the formation of cold and dense gas in colliding flows of diffuse and warm gas. Aims. The objective of this study is to better understand the process of structure formation in a colliding flow and to describe the effects of matter entering the disk on the interstellar medium. Methods. We conducted Herschel-SPIRE observations of the Draco nebula. The clumpfind algorithm was used to identify and characterize the small-scale structures of the cloud. Results. The high-resolution SPIRE map reveals the fragmented structure of the interface between the infalling cloud and the Galactic layer. This front is characterized by a Rayleigh-Taylor (RT) instability structure. From the determination of the typical length of the periodic structure (2.2 pc) we estimated the gas kinematic viscosity. This allowed us to estimate the dissipation scale of the warm neutral medium (0.1 pc), which was found to be compatible with that expected if ambipolar diffusion were the main mechanism of turbulent energy dissipation. The statistical properties of the small-scale structures identified with clumpfind are found to be typical of that seen in molecular clouds and hydrodynamical turbulence in general. The density of the gas has a log-normal distribution with an average value of 10 3 cm −3 . The typical size of the structures is 0.1−0.2 pc, but this estimate is limited by the resolution of the observations. The mass of these structures ranges from 0.2 to 20 M and the distribution of the more massive structures follows a power-law dN/dlog(M) ∼ M −1.4 . We identify a mass-size relation with the same exponent as that found in molecular clouds (M ∼ L 2.3 ). On the other hand, we found that only 15% of the mass of the cloud is in gravitationally bound structures. Conclusions. We conclude that the collision of diffuse gas from the Galactic halo with the diffuse interstellar medium of the outer layer of the disk is an efficient mechanism for producing dense structures. The increase of pressure induced by the collision is strong enough to trigger the formation of cold neutral medium out of the warm gas. It is likely that ambipolar diffusion is the mechanism dominating the turbulent energy dissipation. In that case the cold structures are a few times larger than the energy dissipation scale. The dense structures of Draco are the result of the interplay between magnetohydrodynamical turbulence and thermal instability as self-gravity is not dominating the dynamics. Interestingly they have properties typical of those found in more classical molecular clouds.

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Physical Properties and Galactic Distribution of Molecular Clouds Identified in the Galactic Ring Survey

Cited by 400 publications

References 72 publications

The prevalence of star formation as a function of Galactocentric radius

The prevalence of star formation as a function of Galactocentric radius

GMC Collisions as Triggers of Star Formation. II. 3D Turbulent, Magnetized Simulations

Structure formation in a colliding flow: The Herschel view of the Draco nebula

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