The structure of clouds in the Galactic Halo

We have been measuring HI in the Galactic halo using the 100m Green Bank Telescope (GBT) at an angular resolution of 9'. The sensitivity of this instrument is so high that even for modest integration times of a few seconds it can be seen that much of the halo HI in the inner Galaxy is organized into cloud-like structures whose velocity appears to be dominated by Galactic rotation.

A region above the plane near longitude 35 degrees has received particular attention because it appears to contain a long `whisker' of HI extending from low latitudes to b > 15 deg at a distance of about 7 kpc from the Sun. The GBT observations show that much of the gas in the whisker is in clouds with masses in HI of several tens of solar masses. At this location clouds can be found more than 2 kpc above the plane. We will discuss the physical properties of the clouds and their relationship to other species. Because this set of clouds lies in an ordered structure it is possible that they have been transported up from the plane rather than been formed at their current altitudes. There is an exception for one cloud, though, which appears to be associated with a star and may be part of a different phenomenon entirely.

The National Radio Astronomy Observatory is operated by Associated Universities, Inc., under a cooperative agreement with the National Science Foundation.