Molecular Cloud Distribution in the Outer Galaxy

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Session 60 -- Galactic Structure
Oral presentation, Thursday, January 13, 10:15-11:45, Crystal Forum Room (Crystal City Marriott)

[60.01] Molecular Cloud Distribution in the Outer Galaxy

S.J. Carey (Rensselaer)

We present the results of a survey of a square degree of the outer Galaxy in the ${\rm {}^{12}CO \: J = 1 \rightarrow 0}$ transition using the ${\rm NRAO^{\ast}}$ 12 meter telescope. This survey is the first to be sensitive to small and/or low surface brightness clouds. The survey covers galactocentric radii between 9 and 16 kpc. Although we have chosen a region with a strong HI spiral arm feature, ($l = 76.4^{\circ}$, $b = 1.5^{\circ}$), the survey is not biased towards bright (massive star forming) or giant clouds, and is sensitive to clouds with temperatures of ${\rm < 1K}$ on the ${\rm T^{\ast}_{\tiny R}}$ scale and sizes of ${\rm >= 4 pc}$ at R = 16 kpc. In the survey region, 137 individual clouds have been observed. Two distinct cloud populations, arm and interarm, have been identified. The molecular spiral arm in this region is centered on ${\rm R \approx 13 \, kpc}$ and is ${\rm \sim 1.5 \, kpc}$ wide. As expected, the arm population is dominated by the more massive (${\rm 10^{5} M_{\odot}}$) clouds, while the interarm population consists mostly of small (${\rm diameter < 10 \, pc}$) clouds. The peak line temperatures and linewidths of comparable mass clouds in the two populations are similar, suggesting that the two populations have similar physical characteristics. However, the cloud size distributions of the two populations are very different. This discrepancy may possibly be explained by different formation mechanisms for large and small clouds.

We have also investigated the properties of the clouds as a function of cloud size. We find that ${\rm \sim 30 \%}$ of the mass (${\rm 1.1 \times 10^{6} \, M_{\odot}}$) of the survey region is contained in small clouds. Implications of this finding on estimates of the total molecular mass of the outer Galaxy will be discussed.

\noindent S. Carey was aided by the travel support program of the Astronomical Society of New York and a Grant-in-Aid of Research from the National Academy of Sciences, through Sigma Xi, The Scientific Research Society.

\noindent ${\rm {}^{\ast}}$ The National Radio Astronomy Observatory is operated by Associated Universities, Inc., under cooperative agreement with the NSF.

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