GHRS and ORFEUS II observations of the highly ionized interstellar medium toward ESO 141-055.
SEMBACH K.R., SAVAGE B.D. and HURWITZ M.
Abstract (from CDS):
We present Goddard High Resolution Spectrograph (GHRS) and ORFEUS II measurements of Si IV, C IV, N V, and O VI absorption in the interstellar medium of the Galactic disk and halo toward the nucleus of the Seyfert galaxy ESO 141-055. The high-ionization absorption is strong, with line strengths consistent with the spectral signature expected for hot (T≳105 K) collisionally ionized gas in either a ``Galactic fountain'' or an inhomogeneous medium containing a mixture of conductive interfaces and turbulent mixing layers. The total O VI column density of ~1015 cm–2 suggests that the scale height of O VI is large (≳3 kpc) in this direction. Models of the C IV velocity distribution along the sight line are consistent with a large scale height for the highly ionized gas. Comparison of the high ion column densities with measurements for other sight lines indicates that the highly ionized gas distribution is patchy. The amount of O VI perpendicular to the Galactic plane varies by at least a factor of ∼4 among the complete halo sight lines thus far studied. In addition to the high ion absorption, lines of low-ionization species are also present in the spectra. With the possible exception of Ar I, which may have a lower than expected abundance resulting from partial photoionization of gas along the sight line, the absorption strengths are typical of those expected for the warm, neutral interstellar medium. The sight line intercepts a cold molecular cloud with N(H2)~1019 cm–2. The cloud has an identifiable counterpart in IRAS 100 µm emission maps of this region of the sky. We detect a Lyα absorber associated with ESO 141-055 at z=0.03492. This study presents an enticing glimpse into the interstellar and intergalactic absorption patterns that will be observed at high spectral resolution by the Far Ultraviolet Spectroscopic Explorer.