We have obtained high resolution FUSE and HST/STIS echelle observations of the quasar PG 1116 + 215 ( z _ { \mathrm { em } } = 0.1765 , l = 223 \fdg 36 , b = +68 \fdg 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 < v _ { \mathrm { LSR } } < +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 ( v _ { \mathrm { LSR } } \approx - 44 , -7 , +56 km s ^ { -1 } ) , and two at high velocities ( v _ { \mathrm { LSR } } \approx + 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 v _ { \mathrm { LSR } } \approx 10 km s ^ { -1 } , the expected velocity of halo gas co-rotating with the Galactic disk , and v _ { \mathrm { LSR } } \approx + 184 km s ^ { -1 } , the velocity of the higher velocity HVC .
The velocity coincidence of both low and high ionization species in the v _ { \mathrm { LSR } } \approx + 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 ] \sim - 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 the extragalactic UV background , we estimate the cloud has a density of 10 ^ { -2.7 } cm ^ { -3 } with a thermal pressure p / k \approx 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 .