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

Abstract

We have obtained high resolution FUSE and HST/STIS echelle observations of the quasar PG 1116+215. The semi-continuous coverage of the ultraviolet spectrum over the wavelength range 916-2800 provides detections of Galactic and high velocity cloud (HVC) absorption over a wide range of ionization species over the local standard of rest velocity range -100 - +200 km/s. The high dispersion of these spectra (6.5-20 km/s) reveals that low ionization species consist of five discrete components, three at low-to-intermediate velocities, and two at high velocities (v = +100, +184 km/s). Over the same velocity range, the higher ionization species show continuous absorption with column density peaks at v = +10 km/s +184 km/s. The absorption kinematics of the v=+184 km/s HVC suggest a scenario in which a low-ionization cloud of gas is streaming through a hot external medium that is stripping gas from this cloud. Using the OI and HI column densities, we estimate [O/H]=-0.66, with a substantial uncertainty due to saturation of the HI Lyman series. If the ionization of the cloud core is photonionization by the extragalactic UV background, we estimate the cloud has a density of 10^-2.7 cm^-3. If photons escaping the Galactic disk are also included, the density could be higher by nearly 2 dex. In either case, the relative abundances of O, Si, and Fe in the cloud core are readily explained by a solar pattern. Magellanic Stream gas is a possible origin for this gas and is consistent with the location of the HVC on the sky, as well as its high positive velocity, the ionization, and metallicity. [Abridged]