Highly Ionized Gas in the Galactic Halo and the High Velocity Clouds Toward PG 1116 + 215 1

We have obtained high resolution FUSE and HST/STIS echelle observations of the quasar PG1116 + 215 (zem = 0.1765, l = 223. ◦36, b = +68. ◦21). The semi-continuous coverage of the ultraviolet spectrum over the wavelength range 916–2800 Å provides detections of Galactic and high velocity cloud (HVC) absorption over a wide range of ionization species: H I, C II-IV, N I-II, O I, O VI, Mg II, Si II-IV, P II, S II, and Fe II over the velocity range −100 < vLSR < +200 km s −1. The high dispersion of these spectra (6.5-20 km s−1) reveals that low ionization species consist of five discrete components: three at lowand intermediate-velocities (vLSR ≈ −44,−7,+56 km s −1), and two at high velocities (vLSR ≈ +100,+184 km s −1). Over the same velocity range, the higher ionization species (C III-IV, O VI, Si IV) those with ionization potentials larger than 40 eV show continuous absorption with column density peaks at vLSR ≈ 10 km s−1, the expected velocity of halo gas co-rotating with the Galactic disk, and vLSR ≈ +184 km s −1, the velocity of the higher velocity HVC. The velocity coincidence of both low and high ionization species in the vLSR ≈ +184 km s −1 HVC gas suggests that they arise in a common structure, though not necessarily in the same gaseous phase. The absorption structure in the high ionization gas, which extends to very low velocities, suggests a scenario in which a moderately dense cloud of gas is streaming away from the Galaxy through a hot external medium (either the Galactic halo or corona) that is stripping gas from this cloud. The cloud core produces the observed neutral atoms and low-ionization species. The stripped material is the likely source of the high-ionization species. Among the host of collisionally-ionized non-equilibrium models, we find that shock-ionization and conductive interfaces can account for the column density ratios of high ionization species. The nominal metallicity of the neutral gas, using the O I and H I column densities is [O/H] ∼ −0.66, with a substantial uncertainty due to the saturation of the H I Lyman series in the FUSE band. The ionization of the cloud core is likely dominated by photons, and assuming the source of ionizing photons is Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 Department of Astronomy, University of Massachusetts, Amherst, MA 01003 Department of Astronomy, University of Wisconsin-Madison, 475 N. Charter Street, Madison, WI 53706