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Session 42 - Molecular Clouds: Models and Observations.
Display session, Tuesday, June 11
Great Hall,
We took advantage of innovations at mm-wave telescopes to map the large-scale emission from dense gas in the central 630 pc of the Milky Way. I use the HCN/CO ratio and the HCN 3-2/1-0 ratio to determine the densities of the molecular gas in the Galactic center.
Since stars form solely in the dense cores of molecular clouds, observing the emission from molecules that require high densities for excitation is important for estimating the physical properties (density, mass, temperature) of star-forming clouds in galaxies. Most extragalactic molecular line studies focus on ``starbursts,'' galaxies with unusually high star formation rates, but knowledge of the gas conditions in normal galaxies is essential for placing the starburst results into proper context. Unfortunately, the spatial resolution of typical extragalactic observations is poor (>\,0.3 kpc), and to compare properly the emission from different sources, we must examine them on the same spatial scales. Therefore a sensitive map of a normal galaxy, such as the Milky Way, at high resolution is critical, but to achieve the necessary map scales requires surveying a large angular area.
The Milky Way spectra, when convolved to the spatial resolution of extragalactic data, are very similar to those of other galaxies. However, with our sensitivity, we discern two distributions to the dense gas emission: a bright narrow feature from dense gas in the Sagittarius clouds, and a broad faint component from diffuse gas perhaps in the Galactic disk. Starburst galaxies have higher average densities, a higher fraction of their mass is dense, and the starburst area is larger than the extent of the Sagittarius region. The density derived from the convolved Milky Way spectra equals the mean density found from modeling each map position. Therefore, this analysis yields the average gas properties in galaxies, despite poor spatial resolution.
