  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.
