The elongations and supersonic motions of molecular clouds

New ¹³CO data from the BU-FCRAO Milky Way Galactic Ring Survey (GRS) are analyzed to elucidate the shape and internal motions of molecular clouds. For a sample of more than 500 molecular clouds, we find that they are preferentially elongated along the Galactic plane. On the other hand, their spin axes are randomly oriented. We therefore conclude that the elongation is not supported by internal spin but by internal velocity anisotropy. It has been known that some driving mechanisms are necessary to sustain the supersonic velocity dispersion within molecular clouds. The mechanism for generating the velocity dispersion must also account for the preferred elongation. This excludes some driving mechanisms, such as stellar winds and supernovae, because they do not produce the systemic elongation along the Galactic plane. Driving energy is more likely to come from large-scale motions, such as Galactic rotation.