2005ApJ...630..332F

We study the physical conditions, elemental abundances, and kinematics of the high-velocity clouds (HVCs) along the sight lines toward active galaxies HE 0226-4110 and PG 0953+414 using Hubble Space Telescope Space Telescope Imaging Spectrograph and Far Ultraviolet Spectroscopic Explorer data. No 21 cm H I emission is detected in these clouds, but our observations reveal multiple components of HVC absorption in lines of H I, C II, C III, C IV, O VI, Si II, Si III, and Si IV in both directions. We investigate whether photoionization by the extragalactic background radiation or by escaping Milky Way radiation can explain the observed ionization pattern. We find that photoionization is a good explanation for the C II, C III, Si II, and Si III features but not for the O VI or C IV associated with the HVCs, suggesting that two principal phases exist: a warm (T~10⁴ K), photoionized phase and a hotter (T=1-3x10⁵ K), collisionally ionized phase; the broader line widths of the high ions are consistent with this multiphase hypothesis. The warm HVCs toward HE 0226-4110 have high levels of ionization (97%-99%) and metallicities ([Z/H] between -0.9 and -0.4) close to those in the Magellanic Stream, which lies 11° away on the sky at similar velocities. These HVCs may well be stripped fragments of the Stream that have been ionized by the pervading radiation field; they have thermal pressures that would place them close to equilibrium in a fully ionized 10⁶ K Galactic corona with n_H=4-9x10^–5/cm3 at 50 kpc. The warm HVCs seen at -146 and 125 km/s toward PG 0953+414 have [Z/H]=-0.6±0.2 and -0.8±0.2, respectively, suggesting they are not formed from purely Galactic material. A minisurvey of the hot, collisionally ionized HVC components seen here and in five other sight lines finds that in 11/12 cases, the high ions have kinematics and ionic ratios that are consistent with an origin in conductive interfaces, where energy flows into the HVCs from a hot surrounding medium and produces O VI- and C IV-bearing boundary layers. However, the broad absorption wing on the O VI profile toward PG 0953+414 is not completely explained by the interface scenario. This feature may be tracing the outflow of hot gas into the Milky Way halo as part of a Galactic fountain or wind.