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High Velocity Clouds in the Galaxy

Published online by Cambridge University Press:  07 August 2017

Felix J. Lockman
Felix J. Lockman*
Affiliation:
National Radio Astronomy Observatory, P.O. Box 2, Green Bank, WV 24944 USA ([email protected])

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Early observers measuring 21 cm HI profiles away from the Galactic plane found not only the emission near zero velocity expected from gas in the immediate vicinity of the Sun, but also occasional emission at velocities reaching several hundred km s−1. It seemed unlikely that these spectral lines could come from gas in normal galactic rotation (they are sometimes found at |b| > 45°), and so began the puzzle of “high-velocity clouds” (HVCs). The early result that all HVCs had negative velocity implying that they were infalling was soon shown to be incorrect with the discovery of many positive velocity clouds in the southern hemisphere. Attempts to determine the distance to HVCs by searching for them in absorption against stars yielded only lower limits, typically > 1 kpc. By 1984 several large-scale surveys had established that a significant fraction of the sky was covered with high velocity HI (e.g., Oort, 1966; Giovanelli, 1980). A recent major review is by Wakker (1991a; see also van Woerden, 1993). For this brief presentation to a specialized audience, I will concentrate on issues that may be relevant to the topic of stellar populations.

Type
Session 3: Open Clusters and the Galactic Disk
Information
Symposium - International Astronomical Union , Volume 164: Stellar Populations , 1995 , pp. 129 - 132
Copyright
Copyright © Kluwer 1995 

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