Highly ionized high-velocity clouds: Hot intergalactic medium or galactic halo?

We use spectroscopic data from the Hubble Space Telescope (HST) and Far Ultraviolet Spectroscopic Explorer ( FUSE) to study the wide range of ionization states of the "highly ionized high-velocity clouds'' (HVCs). Studied extensively in O vi absorption, these clouds are usually assumed to be infalling gas in the Galactic halo at distances less than 50 kpc. An alternative model attributes the O vi ( and O VII X-ray absorption) to cosmological structures of low-density, shock-heated intergalactic gas, distributed over 1 - 3 Mpc surrounding the Milky Way. The latter interpretation is unlikely, owing to the enormous required mass of gas (4 x 10(12) M-.). Our detection, in 9 of 12 sight lines, of low-ionization stages ( C II /III /IV; Si II/III/IV) at similar high velocities as O vi requires gas densities far above that (n(H) approximate to 5 x 10(-6) cm(-3)) associated with the warm-hot intergalactic medium ( WHIM). These HVCs are probably cooling, multiphase gas in the Galactic halo, bow shocks, and interfaces between clouds falling through a hot, rotating gaseous halo. The velocity segregation of these HVCs in Galactic coordinates is consistent with a pattern in which infalling clouds reflect the sense of Galactic rotation, with peculiar velocities superposed.