G28.17+0.05, A Giant Atomic/Molecular Cloud
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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.
To appear in "Proceedings of Seeing Through the Dust, The Detection of HI and the Exploration of the ISM in Galaxies", published by the Astronomical Society of the Pacific, editors Russ Taylor, Tom Landecker, and Tony Willis
To appear in "Proceedings of Seeing Through the Dust, The Detection of HI and the Exploration of the ISM in Galaxies", published by the Astronomical Society of the Pacific, editors Russ Taylor, Tom Landecker, and Tony Willis