The Elongations and Supersonic Motions of Molecular Clouds

New 13CO data from the BU-FCRAO Milky Way Galactic Ring Survey (GRS) areanalyzed to understand the shape and internal motions of molecular clouds. Fora sample of more than five hundred molecular clouds, we find that they arepreferentially elongated along the Galactic plane. On the other hand, theirspin axes are randomly oriented. We therefore conclude that the elongation isnot supported by internal spin but by internal velocity anisotropy. It has beenknown that some driving mechanisms are necessary to sustain the supersonicvelocity dispersion within molecular clouds. The mechanism for generating thevelocity dispersion must also account for the preferred elongation. Thisexcludes some driving mechanisms, such as stellar winds and supernovae, becausethey do not produce the systemic elongation along the Galactic plane. Drivingenergy is more likely to come from large scale motions, such as the Galacticrotation.Comment: 6 files; 4 figures; accepted for Ap