The Shapes and Supersonic Motions of Molecular Clouds

The energy source and mechanism for driving the supersonic motions in molecular clouds remain unknown. The unknown driving mechanism must exert an influence on the shape of molecular clouds [5]. We analyzed a sample of more than 500 molecular clouds identified with the BU-FCRAO Galactic Ring Survey (YRS) data, and found that they are preferentially elongated along the Galactic plane. Their spin axes are randomly oriented; therefore, we conclude that the elongation is supported by internal velocity anisotropy, but not by spin. The mechanism for driving the supersonic velocity dispersion must also account for the preferred elongation. This excludes some suggested mechanism, such as stellar winds and supernovae, because they do not produce the systemic elongation. Driving energy is more likely to come from large-scale motions, such as Galactic rotation.