To gain insight into four highly ionized high-velocity clouds ( HVCs ) discovered by Sembach et al .
( 1999 ) , we have analyzed data from the Hubble Space Telescope ( HST ) and Far Ultraviolet Spectroscopic Explorer ( FUSE ) for the PKS 2155 - 304 and Mrk 509 sight lines .
We measure strong absorption in O vi and column densities of multiple ionization stages of silicon ( Si II/III/IV ) and carbon ( C II/III/IV ) .
We interpret this ionization pattern as a multiphase medium that contains both collisionally ionized and photoionized gas .
Toward PKS 2155 - 304 , for HVCs at -140 and -270 km s ^ { -1 } , respectively , we measure log N ( O vi ) = 13.80 \pm 0.03 and log N ( O vi ) = 13.56 \pm 0.06 ; from Lyman series absorption , we find log N ( H i ) = 16.37 ^ { +0.22 } _ { -0.14 } and 15.23 ^ { +0.38 } _ { -0.22 } .
The presence of high-velocity O vi spread over a broad ( 100 km s ^ { -1 } ) profile , together with large amounts of low-ionization species , is difficult to reconcile with the low densities , n _ { e } \approx 5 \times 10 ^ { -6 } cm ^ { -3 } , in the collisional/photoionization models of Nicastro et al .
( 2002 ) , although the HVCs show a similar relation in N ( Si iv ) / N ( C iv ) versus N ( C ii ) / N ( C iv ) as high- z intergalactic clouds .
Our results suggest that the high-velocity O vi in these absorbers do not necessarily trace the WHIM , but instead may trace HVCs with low total hydrogen column density .
We propose that the broad high-velocity O vi absorption arises from shock ionization , at bowshock interfaces produced from infalling clumps of gas with velocity shear .
The similar ratios of high ions for HVC Complex C and these highly ionized HVCs suggest a common production mechanism in the Galactic halo .