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M. A. Kolpak, J. M. Jackson, T. M. Bania (Boston University IAR)
HI absorption spectra measured against bright background continuum sources offer a simple way to measure the HI optical depth as a function of galactocentric radius. We have observed the 21 cm HI line in absorption toward 100 bright, compact radio continuum sources in the first Galactic quadrant with the VLA in C-array. The HI absorption spectra were measured directly, without position switching. In C-array, the VLA is not sensitive to structure on scales larger than 10 arcminutes at L-band. We therefore do not detect the large-scale diffuse HI emission that has plagued single-dish studies of this kind. We have determined the average radial profile of 21 cm HI optical depth in the inner Galaxy. We measured the optical depth of HI absorption in each spectral channel, and each channel velocity was then transformed to a galactocentric radius using the Clemens (1985) rotation curve. There is a broad maximum between radii of 4 and 8.5 kpc, where the average optical depth exceeds 0.5. This feature corresponds to the 5 kpc molecular ring, the largest concentration of molecular gas in the inner Galaxy. We suggest that the peak in HI opacity at these radii is due to the presence of cold atomic gas associated with molecular clouds.
This work was supported in part by NSF grant AST-9800334.