Distribution and Physical Properties of Arm and Interarm Molecular Clouds in the Outer Galaxy

Understanding the nature of the distribution of molecular material as a function of interstellar environment is an important step in understanding cloud formation and evolution. I have conducted an unbiased, high sensitivity, 60^{''} sampled survey of the outer Galaxy in the ¹²CO (J = 1 to 0) transition. Two different cloud populations, arm and interarm, have been identified. The arm population coincides with the 13 kpc arm identified by Mead & Kutner (1988) and is dominated by massive clouds. The interarm population consists almost entirely of small clouds which are scattered throughout the survey volume. The small (10^3-10^4 Modot) clouds are more underluminous in CO than previously investigated clouds by a factor of four or more. The kinetic temperatures of the clouds are not a strong function of cloud environment and range between 5-12 K. The size-linewidth relationship is examined for this sample of clouds, and no correlation is found between cloud size and the observed linewidth of the ¹²CO (J = 1 to 0) transition. A major implication of this measured cloud distribution is that a significant portion (~30%) of the molecular mass of the outer Galaxy is contained in small cold clouds. As small clouds in the outer Galaxy appear to be underluminous when compared to their larger inner Galaxy counterparts, virial mass estimates are preferred over estimating cloud masses using the empirical relationship between H_2 column density and the integrated intensity of the ^ {12}CO (J = 1 to 0) line. Estimates of the total molecular mass are interpreted using the observed small cloud distribution and inadequacy of the N(H_2) to I_CO relationship to determine cloud masses for clouds in the outer Galaxy. Previous total mass estimates (Wouterloot et al. (1990) and Digel (1991)) of the outer Galaxy should be scaled upward by a factor 2-3. My best estimate of the total molecular mass of the outer Galaxy is between 1 and 2 times 10^9 Modot. Cold molecular material in the outer Galaxy is searched for, but not found.