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E.D. Miller, J.N. Bregman, P. Seitzer (U. Michigan)
Gaseous halos play several critical roles in the structure and evolution of spiral galaxies, however our knowledge of these key components is at an early stage. While the presence of emission lines such as H\alpha indicate ionization in the halos of several edge-on galaxies, little is known about the volume filling factor and density of this warm component. We present HST/STIS observations of a quasar projected at a height of z = 5 kpc behind the inner halo of the nearby edge-on spiral galaxy NGC 891. The spectra show strong resonance absorption by MgII and FeII at the redshift of the galaxy, indicating a significant column density of low-ionization material. We determine a total hydrogen column density of N(H) ~6 \times 1020 cm-2; when combined with the published H\alpha emission measure, we obtain a volume filling factor near unity for the ionized (T ~104 K) component of the halo gas. We discuss the implications of this result on the gas density and pressure, which allow us to determine the amount of gas that cools at this height above the disk.