Angular Momentum in Giant Molecular Clouds. I. The Milky Way

We present a detailed analysis comparing the velocity fields in molecular clouds and the atomic gas that surrounds them in order to address the origin of the gradients. To that end, we present first-moment intensity-weighted velocity maps of the molecular clouds and surrounding atomic gas. The maps are made from high-resolution ¹³CO observations and 21 cm observations from the Leiden/Argentine/Bonn Galactic H I Survey. We find that (1) the atomic gas associated with each molecular cloud has a substantial velocity gradient—ranging from 0.02 to 0.07 km s^-1 pc^-1—whether or not the molecular cloud itself has a substantial linear gradient. (2) If the gradients in the molecular and atomic gas were due to rotation, this would imply that the molecular clouds have less specific angular momentum than the surrounding H I by a factor of 1-6. (3) Most importantly, the velocity gradient position angles in the molecular and atomic gas are generally widely separated—by as much as 130° in the case of the Rosette molecular cloud. This result argues against the hypothesis that molecular clouds formed by simple top-down collapse from atomic gas.