High-velocity cloud

A concentration of neutral hydrogen whose velocity differs substantially from the Milky Way’s disk rotation. High-velocity clouds are identified by their anomalous Doppler-shifted 21-centimeter emission.

21-centimeter line: The radio spectral line produced by hyperfine transitions in neutral hydrogen atoms. Its Doppler shift reveals the line-of-sight velocities used to identify high-velocity clouds.
Magellanic Stream: A long trail of gas extending from the Magellanic Clouds through the Milky Way’s halo. It is a prominent source of high-velocity gas associated with interactions between satellite galaxies and the Milky Way.
Galactic accretion: The acquisition of gas by a galaxy from its surroundings or orbiting companions. Infalling high-velocity clouds may replenish gas consumed by star formation in the disk.
Intermediate-velocity cloud: A Milky Way gas cloud with a velocity between typical disk gas and high-velocity clouds. Its smaller velocity deviation distinguishes it from the anomalous population considered here.
Doppler effect: The change in a wave’s observed frequency caused by relative motion between source and observer. It converts each cloud’s shifted hydrogen-line frequency into a measured radial velocity.
Smith Cloud: A massive, high-velocity gas cloud moving toward the Milky Way’s disk. Its measured trajectory makes it a well-studied case for testing whether clouds can deliver gas to the disk.
Milky Way halo: The extended region around the Milky Way containing its dark matter, stars, and hot gas. Cloud locations and motions probe the structure and conditions of the Galaxy’s gaseous halo.
High-velocity cloud complex: A large, coherent assemblage of high-velocity gas clouds sharing related positions and motions. Complexes group individual clouds into extended structures with potentially common origins.
Neutral hydrogen: Hydrogen atoms that are not ionized, commonly observed in space through their 21-centimeter emission. Its emission traces the gas that defines these clouds observationally.
Galactic fountain: A cycle in which gas expelled from a galaxy’s disk later cools and falls back. Some clouds may form when material driven out of the Milky Way returns through its halo.

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H I regionBroader topic: These H I structures expose gas accretion and unusual flows around the Milky Way.