High-Velocity Clouds

The high-velocity clouds (HVCs) are gaseous objects that do not partake in differential galactic rotation, but instead have anomalous velocities. They trace energetic processes on the interface between the interstellar material in the Galactic disk and intergalactic space. Three different processes appear to be responsible for the formation of HVCs. First, supernovae in the Galactic disk create hot gas that vents into the halo, cools and rains back down, in a process generically termed the "Galactic Fountain," in which gas circulates between the disk and halo at a rate of a few M ⊙ yr - 1. This implies that the interstellar medium (ISM) circulates through the halo on timescales of a Gyr. Second, gas streams are formed by tides working on nearby dwarf galaxies (with a possible contribution from ram pressure); this applies specifically to the Magellanic Stream, which was extracted from the Small Magellanic Cloud. Third, low-metallicity clouds are accreting onto the Milky Way, at a present-day rate of about 0.4 M ⊙ yr - 1. Such infall causes the Milky Way to grow and continue forming stars. The source of the infalling material may lie in the cooling of hot (T > 106n K) intergalactic gas that permeates space, or in cold (T <105 K) accretion streams that are theoretically predicted to transport material from intergalactic filaments to galaxies. This chapter describes the observed locations, velocities, and physical conditions of the HVCs. Also included is a discussion of the methods used to derive their distances and metallicities, as well as of the resulting values. Finally, the different origins of the HVCs are discussed.