| Term: | High-velocity clouds |
| Definition: |
A population of neutral or partly ionized gas clouds in the Galactic halo which are seen as high-altitude structures in the atomic hydrogen 21 cm emission at high radial velocities (v_LSR > 100 km/sec). They have substantial neutral column densities (> 10^19 cm^-2) and their metallicities range from 0.1 to about 1.0 times solar. The distances to the majority of them remain unknown. They may represent the continuing infall of matter onto the Local Group. |