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Session 77 - Normal Galaxies.
Oral session, Thursday, June 13
Union Theater,
We summarize the properties of the emission from the 3 mm transitions of HCN, CS, and CO on size scales of \ga 100 pc in the Milky Way. This study combines new observations of individual GMCs and the Milky Way plane with published studies of the central 500 pc of the Galaxy to present a view of the Milky Way that can serve as a basis for comparison with other galaxies. The fraction of emission from dense gas tracers is a strong function of location in the Milky Way: in the bulge, I_HCN/I_CO = 0.081 \pm 0.004, in the plane, I_HCN/I_CO = 0.026 \pm 0.008 on average, and over the full extent of nearby GMCs, I_HCN/I_CO = 0.014 \pm 0.020. (Formal uncertainties are quoted; these underestimate the uncertainties due to absolute calibration errors.) The same sort of trend is seen in I_CS/I_CO. The low intensities of the HCN and CS emission in the plane suggest that these lines are produced by gas at moderate densities; they are thus not like the emission produced by the dense, pc-scale star forming cores in nearby GMCs. The difference between the bulge and disk ratios in the Milky Way is likely to be caused by a combination of higher kinetic temperatures as well as higher average densities in the bulge of the Milky Way.
