Giant Molecular Clouds are More Concentrated to Spiral Arms than Smaller Clouds

From our catalog of Milky Way molecular clouds, created using a temperature thresholding algorithm on the Bell Laboratories CO Survey, we have extracted two subsets: (1) Giant Molecular Clouds (GMCs), clouds that are definitely larger than 10 M⊙, even if they are at their “near distance”, and (2) clouds that are definitely smaller than 10 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 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%) 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, whereas the median separation between smaller clouds and the nearest spiral arm is 5.5± 0.2 km s. Subject headings: Galaxy: structure—ISM: clouds—ISM: molecules