Ultraviolet absorption line studies of the Galactic interstellar medium with the Goddard High Resolution Spectrograph

Summary

The high spectral resolution and high signal to noise capabilities of the Goddard High Resolution Spectrograph (GHRS) have permitted very accurate measurements of the gas phase abundances and physical conditions in interstellar clouds found in the Galactic disk and low halo and of the matter in several Galactic high velocity clouds. The interstellar gas phase abundances provide important clues about the composition of dust grain mantles and cores, and about the origins of intermediate and high velocity gas in the Galactic disk and halo. The processes that circulate gas from the disk into the low halo do not destroy dust grain cores. The gas in Complex C in the direction of Mrk 290 has a metallicity of 0.089 ± 0.024 solar, which implies the accretion of low metallicity gas by the Milky Way at a rate per unit area sufficient to solve the long standing Galactic G-dwarf problem. GHRS studies of interstellar Si IV, C IV, and N V absorption toward stars and AGNs have yielded measures of the 3 to 5 kpc extension of hot gas into the halo of the Milky Way. The GHRS results coupled with new measurements from the Far-Ultraviolet Spectroscopic Explorer (FUSE) satellite of O VI absorption by hot halo gas permit a study of the physical conditions in the hot Galactic Corona originally envisioned by Lyman Spitzer in his classic 1956 paper “On a Possible Interstellar Galactic Corona.”