High-velocity Cloud
These interstellar clouds possess a velocity higher than can be explained by the rotation of the Milky Way. By definition, these clouds must have a vlsr greater than 90 km s−1, where vlsr is the local standard rest velocity. They are detected primarily in the 21 cm line of neutral hydrogen, and typically have a lower portion of heavy elements than is normal for interstellar clouds in the Milky Way.
Theories intended to explain these unusual clouds include materials left over from the formation of our galaxy, or tidally-displaced matter drawn away from other galaxies or members of the Local Group. An example of the latter is the Magellanic Stream. To narrow down the origin of these clouds, a better understanding of their distances and metallicity is needed.
High-velocity clouds are identified with an HVC prefix, as with HVC 127-41-330.
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