The Radial Distribution of Cold Atomic Hydrogen in the Galaxy

Abstract

H I absorption spectra measured against bright extragalactic radio continuum sources offer a simple way to measure the optical depth of cold atomic gas as a function of Galactocentric radius. We have observed the 21 cm H I line in absorption toward 54 bright, compact extragalactic radio continuum sources in the first Galactic quadrant with the VLA in C array. We have determined the average radial profile of 21 cm H I optical depth in the first quadrant using the Clemens rotation curve. There is a region of high average optical depth between Galactic radii of 4 and 8 kpc where the average optical depth, , exceeds 0.8. This region also contains most of the molecular gas in the Galaxy. Furthermore, the highest peak in occurs in the region of the 5 kpc molecular ring, the Galaxy's single most prominent molecular feature. Inward of 4 kpc, drops below 0.5, and beyond 8.5 kpc, remains below 0.4. The H I optical depth measurements were used to construct the 21 cm H I velocity-averaged absorption coefficient, , as a function of Galactocentric radius. The average value of exceeds 4.5 km s-1 kpc-1 between 4 and 8 kpc and drops sharply at smaller and larger radii. The highest peak in rises above 15 km s-1 kpc-1 and again corresponds to the 5 kpc molecular ring. Two other large peaks correspond to the Sagittarius and Perseus spiral arms. We suggest that the high H I opacity in the inner Galaxy, particularly between 4 and 8 kpc, is due to the presence of cold atomic gas associated with molecular clouds.