scholarly journals Giant molecular clouds in the galaxy: distribution, mass, size and age

1979 ◽  
Vol 84 ◽  
pp. 35-52 ◽  
Author(s):  
P. M. Solomon ◽  
D. B. Sanders ◽  
N. Z. Scoville
Keyword(s):  
Interstellar Medium ◽  
Molecular Clouds ◽  
Galactic Disk ◽  
Free Fall ◽  
Major Constituent ◽  
Giant Molecular Clouds ◽  
Interstellar Clouds ◽  
Dominant Component ◽  
The Galaxy ◽  
Galactic Distribution

Millimeter wave observations of emission from the CO molecule have become, over the past eight years, the dominant method for determining the physical properties of dense interstellar clouds, composed primarily of molecular hydrogen and for exploring the structure and kinematics of the galactic disk. In this paper we briefly review the CO survey results in the literature (Section 2) and then present new results (Section 3-7) of an extensive 13CO and 12CO survey of the galactic distribution, size, mass and age of molecular clouds. The interpretation of this survey leads to a new picture of the interstellar medium dominated by very massive stable long-lived clouds which we refer to as Giant Molecular Clouds. We find that Giant Molecular Clouds (GMC's) with M = 105–3 × 106M⊙ are a major constituent of the galactic disk, the dominant component of the interstellar medium in the galaxy interior to the sun and the most massive objects in the galaxy. We find that the interstellar medium and star formation are dominated by massive gravitationally bound clouds in which stars and associations are forming but at a very low rate in comparison to the free fall time. The galactic distribution of the molecules as traced by CO emission is interpreted as the distribution of GMC's. As the most massive objects in the galaxy they are also basic to the dynamics of the disk.

scholarly journals Stability of a Collisional Galactic Disk

1996 ◽  
Vol 169 ◽  
pp. 521-522
Author(s):  
E. Griv
Keyword(s):  
Gravitational Field ◽  
Interstellar Medium ◽  
Molecular Clouds ◽  
Galactic Disk ◽  
Time Span ◽  
Solar Neighborhood ◽  
Stellar Disk ◽  
Giant Molecular Clouds ◽  
The Galaxy ◽  
Kinetic Treatment

Until recently, only collisionless models have been investigated in the kinetic treatment of stellar disk stability (e.g., Fridman and Polyachenko [1984]). This is due to the fact that the frequency of ordinary binary stellar gravitational (elastic) encounters in the Galaxy is much smaller than the variation of the gravitational field for the process being studied. However, in the pioneering paper Spitzer and Schwarzschild (1951) proposed a different kind of encounter: interaction of stars with gas clouds of the interstellar medium having a mass of rougly 106M⊙. In recent years this hypothesis was partially confirmed by observations: it was discovered in the Galaxy a few thousand giant molecular clouds of mass Mc ≥ 105M⊙. Other evidence of dynamical relaxation of the star–cloud disk in the solar neighborhood was found by Grivnev and Fridman (1990); the time of relaxation was estimated equal to τ = (2 – 4) × 109 years. Hence the study of collisional star–cloud system is not only of academic interest – on the time span t ≥ 109 years an actual galaxy may be a collisional ensemble of stars and clouds.

scholarly journals Measurements of Isotopic Abundances in Interstellar Clouds

1980 ◽  
Vol 87 ◽  
pp. 397-404 ◽  
Author(s):  
Arno A. Penzias
Keyword(s):  
Molecular Clouds ◽  
General Framework ◽  
Galactic Center ◽  
Galactic Plane ◽  
Giant Molecular Clouds ◽  
Interstellar Clouds ◽  
The Galaxy ◽  
Isotopic Abundances ◽  
Si Isotopes

While an examination of the available data reveals some seemingly contradictory results, a general framework having the following outlines can be put forward:1. With the exception of the two galactic center sources SgrA and SgrB, the relative isotopic abundances exhibited by the giant molecular clouds in our Galaxy exhibit few, if any, significant variations from the values obtained by averaging the data from all these sources.2. The 13C/12C and 14N/15N abundance ratios are ∼130% and ∼150%, respectively, of their terrestrial values throughout the galactic plane and somewhat higher, ∼300%, near the galactic center.3. The 16O/18O and 17O/18O abundance ratios are ∼130% and ∼160%, respectively, of their terrestrial values throughout the Galaxy, although the former may be somewhat lower near the galactic center.4. The S and Si isotopes have generally terrestrial abundances.

scholarly journals The Distribution of Molecular Clouds in Spiral Galaxies

1983 ◽  
Vol 100 ◽  
pp. 35-42
Author(s):  
P. M. Solomon
Keyword(s):  
Millimeter Wave ◽  
Molecular Hydrogen ◽  
Surface Density ◽  
Molecular Clouds ◽  
Spiral Galaxies ◽  
Galactic Disk ◽  
Giant Molecular Clouds ◽  
The Galaxy ◽  
Co Emission ◽  
Half Thickness

The use of millimeter wave CO emission as a tracer of molecular hydrogen in the Galaxy (Scoville and Solomon 1975) showed that most of the H2 unlike HI is concentrated in the inner part of the Galaxy in a “ring” between 4–8 kpc and in the inner 1 kpc. Subsequent surveys (Gordon and Burton 1976, Cohen and Thaddeus 1977, Solomon etal. 1979) confirmed this picture with more extensive data. The molecular interstellar medium was shown to be dominated by Giant Molecular Clouds with individual masses between 105 and 3·106M⊙ (Solomon etal. 1979, Solomon and Sanders 1980). The GMC's confined to a layer with a half thickness of only 60 pc are an important component of the galactic disk, and the most massive objects in the galaxy. They affect the dynamics of the disk by contributing significantly to the surface density and through their individual gravitational interactions with stars.

scholarly journals DIBs, Interstellar Dust, and Extinction

2013 ◽  
Vol 9 (S297) ◽  
pp. 147-152 ◽  
Author(s):  
G. C. Clayton
Keyword(s):  
Interstellar Medium ◽  
Interstellar Dust ◽  
Dust Grains ◽  
Weak Correlation ◽  
Interstellar Clouds ◽  
The Galaxy ◽  
The Relationship

AbstractThe relationship between DIBs and dust is still unknown. The correlation between reddening and DIB strength means that the DIBs are mixed in with the dust and gas in interstellar clouds. The DIBs are relatively stronger in the diffuse interstellar medium than in dense clouds. There is only a weak correlation between the DIBs and the UV extinction parameters including the 2175 Å bump strength and the far-UV rise. In addition, the bump dust grains are sometimes polarized, while the DIBs are not. However, observations of DIBs in the SMC show that when the 2175 Å bump is weak or missing so are the DIBs. Two of the four sightlines that deviate strongly from the CCM UV extinction in the Galaxy show weak DIBs.

scholarly journals The global star formation law: from dense cores to extreme starbursts

2006 ◽  
Vol 2 (S237) ◽  
pp. 331-335
Author(s):  
Yu Gao
Keyword(s):  
Star Formation ◽  
Linear Correlation ◽  
Molecular Clouds ◽  
High Redshift ◽  
Gas Content ◽  
Molecular Gas ◽  
Giant Molecular Clouds ◽  
Active Star ◽  
Dense Cores ◽  
The Galaxy

AbstractActive star formation (SF) is tightly related to the dense molecular gas in the giant molecular clouds' dense cores. Our HCN (measure of the dense molecular gas) survey in 65 galaxies (including 10 ultraluminous galaxies) reveals a tight linear correlation between HCN and IR (SF rate) luminosities, whereas the correlation between IR and CO (measure of the total molecular gas) luminosities is nonlinear. This suggests that the global SF rate depends more intimately upon the amount of dense molecular gas than the total molecular gas content. This linear relationship extends to both the dense cores in the Galaxy and the hyperluminous extreme starbursts at high-redshift. Therefore, the global SF law in dense gas appears to be linear all the way from dense cores to extreme starbursts, spanning over nine orders of magnitude in IR luminosity.

scholarly journals Giant Molecular Clouds in the Galaxy: The Massachusetts-Stony Brook CO Galactic Plane Survey

1987 ◽  
Vol 115 ◽  
pp. 499-499 ◽  
Author(s):  
P. M. Solomon
Keyword(s):  
Molecular Clouds ◽  
Galactic Plane ◽  
Far Infrared ◽  
Formation Mechanisms ◽  
H Ii Regions ◽  
Giant Molecular Clouds ◽  
Star Forming ◽  
The Galaxy ◽  
Two Populations ◽  
Spiral Arm

The CO Galactic Plane Survey consists of 40,572 spectral line observations in the region between 1 = 8° to 90° and b = −1°.05 to +1°.05 spaced every 3 arc minutes, carried out with the FCRAO 14-m antenna. The velocity coverage from −100 to +200 km/s includes emission from all galactic radii. This high resolution survey was designed to observe and identify essentially all molecular clouds or cloud components larger than 10 parsecs in the inner galaxy. There are two populations of molecular clouds which separate according to temperature. The warm clouds are closely associated with H II regions, exhibit a non-axisymmetric galactic distribution and are a spiral arm population. The cold clouds are a disk population, are not confined to any patterns in longitude-velocity space and must be widespread in the galaxy both in and out of spiral arms. The correlation between far infrared luminosities from IRAS, and molecular masses from CO is utilized to determine a luminosity to mass ratio for the clouds. A face-on picture of the galaxy locating the warm population is presented, showing ring like or spiral arm features at R ∼ 5, 7.5 and 9 kpc. The cloud size and mass spectrum will be discussed and evidence presented showing the presence of clusters of giant molecular clouds with masses of 106 to 107 M⊙. The two populations of clouds probably have different star forming luminosity functions. The implication of the two populations for star formation mechanisms will be discussed.

scholarly journals Formation, Evolution and Destruction of Possible DIB Carriers: Dirty Molecular Hydrogen Ice Clusters

2013 ◽  
Vol 9 (S297) ◽  
pp. 381-382
Author(s):  
D. K. Lynch ◽  
L. S. Bernstein ◽  
F. O. Clark
Keyword(s):  
Interstellar Medium ◽  
Single Molecule ◽  
Molecular Hydrogen ◽  
Molecular Clouds ◽  
Single Layer ◽  
Molecular Clusters ◽  
Electronic Transitions ◽  
Giant Molecular Clouds ◽  
Diffuse Interstellar Bands ◽  
Optical Photons

AbstractWe suggest that the diffuse interstellar bands (DIBs) are absorption lines arising from electronic transitions in molecular clusters primarily composed of a single molecule, atom, or ion (“seed”), embedded in a single-layer shell of H2 molecules (Bernstein et al. 2013). We refer to these clusters as CHCs (Contaminated H2 Clusters). CHCs arise from cm-sized, dirty H2 ice balls, called CHIMPs (Contaminated H2 Ice Macro-Particles), formed in cold, dense, Giant Molecular Clouds (GMCs), and later released into the interstellar medium (ISM) upon GMC disruption. Absorption by the CHIMP of a UV photon releases CHCs. CHCs produce DIBs when they absorb optical photons. When this occurs, the absorbed photon energy disrupts the CHC.

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