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

Abstract

We have obtained high-resolution Far Ultraviolet Spectroscopic Explorer (FUSE) and Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) echelle observations of the quasar PG 1116 + 215 (zem = 0.1765, l = 22336, b = +6821). The semicontinuous 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 km s-1 < 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 low and 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 corotating 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 nonequilibrium 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 caused by 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 the extragalactic UV background, we estimate the cloud has a density of 10-2.7 cm-3 with a thermal pressure p/k ≈ 24 cm-3 K. If photons escaping the Galactic disk are also included (i.e., if the cloud lies closer than the outer halo), the density and thermal pressure could be higher by as much as 2 dex. In either case, the relative abundances of O, Si, and Fe in the cloud core are readily explained without departures from the solar pattern. We compare the column density ratios of the HVCs toward the PG 1116+215 to other isolated HVCs as well as Complex C. Magellanic Stream gas (either a diffuse extension of the leading arm or gas stripped from a prior passage) is a possible origin for this gas and is consistent with the location of the high-velocity gas on the sky, as well as its high positive velocity, the ionization, and metallicity.