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

Abstract

We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of the post–asymptotic giant branch star von Zeipel 1128 (l ¼ 42=5, b ¼þ78=7; d ¼ 10:2 kpc; z ¼ 10:0 kpc; vLSR ¼?140 ? 8kms?1), located in the globular cluster Messier 3. The FUSE observations cover the wavelength range 905–1187 A˚ at including absorption from ions associated with the warm neutral, warm ionized, and highly ionized phases of ?20 km s?1 (FWHM) resolution. These data exhibit many photospheric and interstellar absorption lines, 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 compo- nents within the absorption profiles are unresolved by FUSE, a comparison of the velocity distribution of depleted or ionized species with the neutral species suggests that 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 hydro- ated with the warm ionized phase. The warm ionized and neutral media toward von Zeipel 1128 have very gen represents e12%, most likely ?45%, ofthe total hydrogen column along this sight line, most of it associ- 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, ffiffiffi although the O vi velocity dispersion is much higher (? ? p 2 at velocities where lower ionization gas is seen, suggesting that the O vi and WNM/WIM probes are tracing b ¼ 32 km s?1). Virtually all of the O vi is found 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 or with the circumstellar environment of von Zeipel 1128. Neither high-velocity cloud absorption (with |vLSR|e125 km s?1) nor high velocity dispersion gas (with ? ? 60 km s?1) is seen toward von Zeipel 1128.