Theoretical models of high-velocity clouds

Three theories for the origin of the HVC and IVC are considered: galactic fountains, infall of gas removed from dwarf galaxies, and remnants of galaxy formation in the Local Group. Distance is a key discriminator for these theories, so we examine whether the intensity of the H alpha emission can be used to estimate distances. Based upon the H alpha brightness of the Magellanic Stream and HVC Complexes A and M, similar to 1% of the ionizing radiation from the galaxy reaches these clouds, about an order of magnitude lower than that required to ionize the Reynolds layer, but consistent with the clouds having z > 1 kpc. If other HVC have distances of similar to 3-10 kpc, their H alpha brightness should be similar to Complexes A and M, and observations of other HVC should be available soon. We review the state of galactic fountains, of which a key requirement is that breakout of hot gas into the halo occurs. Observational support for breakout is strong both for the Milky Way and other galaxies, and with temperatures that will keep the gas gravitationally bound to the systems. The mass flux rate is difficult to determine directly from the Xray observations because most of the radiation occurs in the inaccessible EUV band. Evidence from several wavelength bands is consistent with a mass flux rate of similar to 1-10 M.yr(-1) Whereas galactic fountains can produce the intermediate velocity gas and some of the HVC, it is unlikely to be able to account for certain features, including some extremely long complexes. We suggest that a second mechanism is essential and that the most likely candidate is the gas that is removed from dwarfs either through tidal effects or ram pressure stripping.