2001ApJ...547..907S

We present Space Telescope Imaging Spectrograph (STIS) and Goddard High Resolution Spectrograph (GHRS) observations of interstellar UV absorption toward HD 177989, a B0 III star at 4.9 kpc in the direction l=17°.8, b=-11°.9. The line of sight passes through the high-latitude ejecta of the Scutum supershell (GS 018-06+44), which is ∼5° in diameter extending ∼7° below the Galactic plane at a kinematic distance of ∼3.5 kpc in the Scutum spiral arm. The observations with the STIS E140H and GHRS echelle B gratings provide far- and middle-UV spectra at resolutions (FWHM) of ∼3 km.s^–1 and a signal-to-noise ratio (S/N) of ∼20:1 to 50:1. The observations reveal strong and broad absorption in the lines of Si IV and C IV centered on LSR velocities of +18 and +42 km.s^–1 and weaker absorption from these ions near -50 and -13 km.s^–1. Weak absorption by N V extends over the full velocity range traced by Si IV and C IV. The +18 km.s^–1 high-ionization absorption likely occurs in gas ∼400 pc below the Sagittarius spiral arm, while the extremely strong +42 km.s^–1 absorption occurs in highly ionized gas in the Scutum supershell at a distance of ∼700 pc below the Galactic plane. The properties of the highly ionized gas associated with the Scutum supershell are similar to the gas found in radio loops I and IV; in both cases there is a strong enhancement in the column density of C IV without a corresponding increase in the column density of N V, which causes N(C IV)/N(N V) to be among the largest measured in the interstellar medium. The low-ionization absorption lines of N I, S II, Si II, and Fe II produce narrow absorption features at +37, +40 km.s^–1 and +55, +60 km.s^–1. The strength and kinematic properties of these absorption features bear no resemblance to those expected for the high-latitude neutral cloud seen in the H I 21 cm line. This may be due to the relatively low angular resolution (FWHM ∼21') of the 21 cm observations. The kinematic relationships among the high-ionization and low-ionization absorption lines observed in the UV suggest a related origin in a hot-warm gas interface region. We are possibly seeing the warm gas in the swept-up shell surrounding a region where hot gas is being vented into the halo. In the warm gas, N(N I)/N(S II) ∼0.01 solar, which implies a similar value for N(H⁰)/[N(H⁰)+N(H⁺)]. The warm neutral and ionized gas in the matter overlying the Scutum supershell has values of Si/S and Fe/S roughly similar to those found in the warm neutral medium of the Galactic disk in the vicinity of the Sun. While there has been grain processing in the ejecta of the Scutum supershell, the processing has not been complete. Based on observations of interstellar C IV and Si IV at high S/N and high resolution toward four very distant stars, we determine that highly ionized gas absorption components occur at a frequency of ∼1 component kpc^–1. The strongest components are associated with lines of sight that pass over or under spiral arms or that pass though Galactic supershells.