Physical Properties and Galactic Distribution of Molecular Clouds identified in the Galactic Ring Survey (1010.2798v1)

Abstract: 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 ISM is around 2.36. This relation, M = (228 +/- 18) R^{2.36+/-0.04}, allows us to derive masses for an additional 170 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. Last, 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.

• The paper reveals a power-law correlation between molecular cloud mass and radius (M = (228±18)R^(2.36±0.04)), underscoring the fractal structure of the interstellar medium.
• It employs 12CO and 13CO line emissions to derive cloud properties and extend mass estimates to 170 additional clouds beyond earlier surveys.
• The research finds that Galactic mass density peaks at 4–5 kpc and CO excitation temperatures decline with radius, suggesting reduced star formation farther from the center.

Analysis of Molecular Clouds in the Galactic Ring Survey

The paper provides an extensive analysis of the physical properties and Galactic distribution of molecular clouds identified in the Galactic Ring Survey (GRS). The authors utilize data from both the University of Massachusetts-Stony Brook (UMSB) survey and the GRS, focusing on the 12 and 13 line emissions of 580 molecular clouds. This work centers on deriving the physical characteristics of these molecular clouds, establishing a power-law relationship between their radii and masses, examining their Galactic distribution, and discussing variations in physical properties with Galactocentric radius.

The analysis reveals a power-law correlation between the radii and masses of the molecular clouds, specifically $M = (228\pm18) R^{2.36\pm0.04}$. This relation implies that the fractal dimension of the interstellar medium (ISM) is approximately 2.36, highlighting the fractal nature of molecular cloud boundaries and interiors. The formulation allows the authors to derive masses for an additional 170 GRS molecular clouds not covered by the UMSB survey, demonstrating a robust methodology for scaling analysis.

A detailed examination of the Galactic surface mass density reveals that it peaks between Galactocentric radii of 4 and 5 kpc. Interestingly, the paper notes the absence of the Perseus arm in molecular gas but confirms enhancements along the Scutum-Crux and Sagittarius arms. This finding suggests a strong correlation between molecular cloud formation and spiral arms, with potential implications for our understanding of molecular cloud dynamics and star formation in the Milky Way.

The research further investigates the CO excitation temperature of molecular clouds and observes a decrease in temperature with increasing Galactocentric radius. This decline is indicative of a potential decrease in the star formation rate as distance from the Galactic center increases, suggesting a radial dependence of star formation activities in the Galaxy.

Theoretical and practical implications of these findings are significant. The confirmation of a correlation between molecular cloud properties and Galactic structure provides insights into the evolution of star-forming regions. The derived physical properties, particularly the detailed catalog of cloud masses and densities, serve as an important reference for numerical simulations of molecular cloud evolution.

In summary, this paper underscores the importance of large-scale surveys like the GRS in advancing our understanding of the structure and distribution of molecular clouds. Future research could extend this analysis to include more comprehensive data from different regions of the Galaxy, thereby refining the proposed models of cloud formation and star-formation activities. The implications for both observational and theoretical astrophysics are profound, potentially shaping future explorations into the dynamic processes governing our Galaxy's evolution.