High Velocity Clouds

On the occasion of the retirement of Ulrich Schwarz, a symposium was held in Groningen in May of 1996, celebrating his contributions to the study of the int- stellar medium, including his work on the high-velocity clouds. The coming together of many specialists in the latter ?eld prompted the idea of compiling a book c- taining their contributions, and summarizing the status of our understanding of the high-velocity cloud phenomenon. This seemed especially worthwhile at the time, since many exciting developments were taking place. After the discovery of some H i clouds with high velocities, about 40 years ago, the subject had been dominated by 21-cm observations of H i emission. Starting in the mid-1980s much progress was being made because of the availability of new instruments, such as large ground-based optical telescopes and UV observatories in space. The connections between the work on high-velocity clouds and other studies of the properties of the (hot) interstellar medium also became clearer.

Magellanic clouds, high-velocity clouds, star formation, infrared emission.

High velocity clouds (HVCs) are collections of gas around the Milky Way galaxy and other galaxies with masses ranging from tens of thousands of solar masses to millions of solar masses. These cool clouds can potentially supply new cool material to the disks of galaxies. Many unanswered questions exist about HVCs; e.g. "How do HVCs form?", "How long can HVCs survive while interacting with the hot ambient material?", and "How do HVCs affect, and how are they affected by, our Galaxy?" Using detailed multi-dimensional numerical simulations, this study focuses on how HVCs interact with their ambient environments and how these interactions affect the cloud's metal concentration, condensation and evaporation rates, and morphology. Using the results of these simulations this study suggests methods to determine the origins of HVCs based on observed metal concentrations, and how initial parameter space affects the condensation of ambient material onto HVCs.