OH as an Alternate Tracer for Molecular Gas: A Study in the W5 Star-Forming Region
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Date
2019-07-31
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Johns Hopkins University
Abstract
Tracing molecular H2 gas in the Galactic interstellar medium is complicated by the fact that diffuse, cold H2 is not detectable. The usual tracer for molecular gas is 12CO(1-0); however, questions have been posed about the universality of CO for this purpose, and evidence has suggested reservoirs of undetected “CO-dark” molecular gas. This dissertation contributes to research into the use of OH 18 cm lines as an alternate tracer for molecular gas. The focus of this dissertation is a survey of the W5 star-forming region using the Green Bank Telescope to determine the structure and quantity of molecular gas in W5, and to compare the properties of W5 to those of a quiescent region according to both tracers. Calculating column densities of OH requires knowledge of the excitation temperature of the observed molecular transi- tion. I have measured excitation temperatures of the OH 18 cm lines in W5 using two distinct methods: the traditional “expected profile” method, and a “continuum background method.” The latter yields more precise results, and demonstrates that the excitation temperature is different for the two 18 cm main lines. Results of the OH survey in W5 are then presented. In W5, the OH and CO trace a similar morphology of molecular gas, in contrast to quiescent regions which can contain CO-dark OH detections. The molecular gas mass traced by OH emission is slightly larger than that traced by CO, but the difference is not considered significant. I propose a volume density-based explanation for the presence or absence of CO-dark molecular gas, and estimate the average volume density for three regions using a diffuse cloud model. The CO-dark gas correlates with lower volume density portions of the qui- escent region, and the highest average volume density occurs in W5. These results suggest that CO-dark molecular gas primarily exists in interstellar space outside of star-forming regions, and that volume density is the primary distinction between the molecular gas in W5 and the quiescent region. I also discuss a novel method based on excitation temperatures for estimating physical conditions in molecular gas without relying on CO.
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Keywords
Galaxy: disk--ISM: molecules--ISM: structure--local interstellar matter--radio lines: ISM--surveys