We use spectroscopic data from HST and FUSE to study the wide range of ionization states of the “ highly ionized high-velocity clouds ” .
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 \times 10 ^ { 12 } ~ { } M _ { \odot } ) .
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 } \approx 5 \times 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 .