Giant Molecular Clouds Are More Concentrated toward Spiral Arms than Smaller Clouds

From our catalog of Milky Way molecular clouds, created using a temperature thresholding algorithm on the Bell Laboratories 13 CO survey, we have extracted two subsets: (1) clouds that are definitely larger than 10 5 M ☉ , even if they are at their "near distance" (i.e., giant molecular clouds [GMCs]), and (2) clouds that are definitely smaller than 10 5 M ☉ , even if they are at their "far distance." The positions and velocities of these clouds are compared to the loci of spiral arms in ( l , v )-space. The radial velocity separation of each cloud from the nearest spiral arm is introduced as a "concentration statistic." Almost all of the GMCs are found near spiral arms. The density of smaller clouds is enhanced near spiral arms, but some clouds (~10% of the smaller clouds) are unassociated with any spiral arm. The median velocity separation between a GMC and the nearest spiral arm is 3.4 ± 0.6 km s -1 , whereas the median separation between smaller clouds and the nearest spiral arm is 5.5 ± 0.2 km s -1 . These separations in radial velocity are composed partly of the velocity dispersion of the cloud populations and partly of velocity differences due to spatial separations between the clouds and the spiral arms in the Galactic rotation field. A simple estimate indicates that the spatial separation component is relatively unimportant. The data are therefore consistent with the hypothesis that most molecular clouds in the Milky Way are spatially located in the spiral arms and that the velocity dispersion of the GMCs within the arms is less than that of the smaller clouds.