The Origin of the High-velocity Clouds

Abstract

There are three leading models for the origin of the high-velocity clouds of neutral hydrogen: galactic fountains, material lost from dwarf galaxies, and the ancient remnants of galaxy formation. The tidal-stripping picture has strong observational support in that the Magellanic Stream is connected to the Magellanic Clouds and such phenomena are seen in other galaxy groups. Also, absorption in at least two sightlines through the Stream shows approximately the metallicity expected for Magellanic Cloud material. The Magellanic Clouds dominate the H i content of the ensemble of smaller galaxies in the Local Group. So, although other Local Group dwarf galaxies may have had their neutral gas removed, it would be a minor component compared to that of the Magellanic Stream. One unsolved issue in this area is whether the presence of other high-velocity gas complexes could be due to gas stripped in previous orbits of the Clouds past the Milky Way.

Galactic fountains are another formation mechanism of intermediate-and high-velocity gas, the main question being the degree of the mass flux involved. This model requires that hot gas, created by supernovae in the disk, escapes into the halo. Theoretical calculations support superbubble breakout, while X-ray, radio, and optical observations reveal the presence of halo gas extending several kiloparsecs above the plane in the Milky Way and in other spiral galaxies. Calculations predict that neutral clouds form from this hot halo gas, both on the way up and on the way down. This mechanism is probably responsible for most of the intermediate-velocity gas and for some of the high-velocity gas (within 10 kpc). We discuss the evidence that clouds have impacted the disk, and the role of cosmic rays populating the halo with hot gas.

In the third model, involving gas that never formed into galaxies, the high-velocity clouds are distributed around the Local Group at distances of up to several hundred kiloparsecs. The total H i mass in this model is comparable to if not greater than the gaseous mass within galaxies, so the clouds could be of cosmological importance. A primary prediction of this model, their large distances, has not been confirmed, but efforts examining this issue are continuing.