Ionized Gas in the First 10 kpc of the Interstellar Galactic Halo

We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of the post-asymptotic giant branch star von Zeipel 1128 (l = 42. 5, b = +78. 7; d = 10.2 kpc, z = 10.0 kpc; vLSR = −140± 8 km s), located in the globular cluster Messier 3. The FUSE observations cover the wavelength range 905 – 1187 Å at ∼ 20 km s (FWHM) resolution. These data exhibit many photospheric and interstellar absorption lines, including absorption from ions associated with the warm neutral, warm ionized, and highly-ionized phases of the interstellar medium along this sight line. We derive interstellar column densities of H I, P II, Ar I, Fe II, Fe III, S III, and O VI, with lower limits for C II, C III, N I, O I, and Si II. Though the individual velocity components within the absorption profiles are unresolved by FUSE, a comparison of the velocity distribution of depleted or ionized species with the neutral species suggests the thick disk material along this sight line is infalling onto the Galactic plane, while material near the plane is seen closer to rest velocities. Ionized hydrogen represents & 12%, most likely ∼ 45%, of the total hydrogen column along this sight line, most of it associated with the warm ionized phase. The warm ionized and neutral media toward von Zeipel 1128 have very similar gas-phase abundances and kinematics: the neutral and ionized gases in this region of the thick disk are closely related. Strong O VI absorption is seen with the same central velocity as the warm ionized gas, though the O VI velocity dispersion is much higher (σ ≡ √ 2b = 32 km s). Virtually all of the O VI is found at velocities where lower-ionization gas is seen, suggesting the O VI and WNM/WIM probes are tracing different portions of the same structures (e.g., the O VI may reside in interfaces surrounding the WNM/WIM clouds). We see no evidence for interstellar absorption associated with the globular cluster Messier 3 itself nor with the circumstellar environment of von Zeipel 1128. Neither high velocity cloud absorption (with |vLSR| & 125 km s) nor high velocitydispersion gas (with σ ∼ 60 km s) is seen toward von Zeipel 1128. Subject headings: ISM: atoms – ISM: structure – ultraviolet: ISM