The Radial Distribution of Cold Atomic Hydrogen in the Galaxy

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, 〈τ(R)〉, exceeds 0.8. This region also contains most of the molecular gas in the Galaxy. Furthermore, the highest peak in 〈τ(R)〉 occurs in the region of the 5 kpc molecular ring, the Galaxy's single most prominent molecular feature. Inward of 4 kpc, 〈τ(R)〉 drops below 0.5, and beyond 8.5 kpc, 〈τ(R)〉 remains below 0.4. The H I optical depth measurements were used to construct the 21 cm H I velocity-averaged absorption coefficient, 〈κ(R)〉, as a function of Galactocentric radius. The average value of 〈κ(R)〉 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 〈κ(R)〉 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.