Outflow vs. Infall in Spiral Galaxies: Metal Absorption in the Halo of
  NGC 891
  Gas accreting onto a galaxy will be of low metallicity while halo gas due to
a galactic fountain will be of near-solar metallicity. We test these
predictions by measuring the metal absorption line properties of halo gas 5 kpc
above the plane of the edge-on galaxy NGC 891, using observations taken with
HST/STIS toward a bright background quasar. Metal absorption lines of Fe II, Mg
II, and Mg I in the halo of NGC 891 are clearly seen, and when combined with
recent deep H I observations, we are able to place constraints on the
metallicity of the halo gas for the first time. The H I line width defines the
line broadening, from which we model opacity effects in these metal lines,
assuming the absorbing gas is continuously distributed in the halo. The
gas-phase metallicities are [Fe/H] = -1.18 +/- 0.07 and [Mg/H] = -0.23
+0.36/-0.27 (statistical errors) and this difference is probably due to
differential depletion onto grains. When corrected for such depletion using
Galactic gas as a guide, both elements have approximately solar or even
supersolar abundances. This suggests that the gas is from the galaxy disk,
probably expelled into the halo by a galactic fountain, rather than from
accretion of intergalactic gas, which would have a low metallicity. The
abundances would be raised by significant amounts if the absorbing gas lies in
a few clouds with thermal widths smaller than the rotational velocity of the
halo. If this is the case, both the abundances and [Mg/Fe] would be supersolar.
