The Orientation and Sizes of Molecular Clouds in the Galaxy

Citation

Redman, Russell Ormond (1982) The Orientation and Sizes of Molecular Clouds in the Galaxy. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/p0pn-sw91. https://resolver.caltech.edu/CaltechETD:etd-09032008-131405

Abstract

In Part I of this dissertation, a technique is introduced to investigate the shapes and orientations of molecular clouds. The clouds are shown to be roughly bar-shaped with their long axes normally aligned within thirty degrees of the axis of the spiral arm in which the cloud is embedded. The long axis is typically three times longer than the two shorter axes, although all axial ratios from one to five appear to be present. These results appear to be independent of cloud size over the range from 15 parsecs to 175 parsecs for the maximum apparent cloud dimension.

In Part II, the size distribution of the CO clouds is measured. Most of the mass of molecular gas in the galaxy resides in the largest clouds, with semimajor axes slightly larger than 100 pc long. The largest cloud sizes appear to be about the same over the entire molecular disk. These very large clouds must have been formed full sized from the intercloud gas, since they cannot have been assembled from smaller clouds. In deriving this distribution a simple and effective technique was developed to resolve the kinematic distance ambiguity.

The concentration of the mass of molecular gas into the largest clouds, and the alignment of the clouds along the spiral arms, both indicate that the clouds are very young. They must be formed directly from the intercloud medium, probably behind a spiral arm shock wave, and cannot grow over long periods of time from smaller clouds. The mechanism forming the clouds from the intercloud gas must have a prefered scale length. Examples of such mechanisms are the Parker magnetic instability, or the collapse of a slowly accumulating dust lane. Similarly, the clouds must be dispersed rapidly at the ends of their lives.

Thesis Files