G28.17+0.05, A Giant Atomic/Molecular Cloud

HI 21 cm observations with the NRAO 140 Foot telescope have revealed a giant HI cloud (G28.17+0.05) in the Galactic plane that has unusual properties. The cloud is 150 pc in diameter, is at a distance of 5 kpc, and contains as much as 100,000 Solar Masses of atomic hydrogen. The cloud consists of a cold core, T~10 K, and a hotter outer envelope, T < 200 K. There is no observable difference in the HI line widths, ~7 km/s, between the core and the envelope. Anomalously-excited 1720 MHz OH emission, with a similar line width, is associated with the core of the cloud. The cloud core also exhibits 12CO and 13CO self-absorption which indicates that most of the cloud mass is in molecules. The total mass of the cloud is greater than 200,000 Solar Masses. The cloud has only a few sites of current star formation. If similar clouds are associated with other observed sites of anomalously-excited 1720 MHz OH emission, there may be as many as 100 more of these objects in the inner galaxy.