Detection of Hot Gas in the Interstellar Medium

Abstract

We present data on the C IV, Si IV, and N V absorption lines in eight disk stars observed by HST (Hubble Space Telescope) with the GHRS (Goddard High Resolution Spectrograph) at intermediate resolution. C IV and Si IV absorption lines are detected in seven target stars, while N V lines are detected in four distant stars and one nearby star. By fitting these absorption lines, we find a strong linear correlation between the column densities of the ions and distance. The average column density ratios of C IV to Si IV and C IV to N V are 4.7 and 3.0, which roughly match the result of lUE surveys. A correlation of the line widths of C IV and Si IV leads to the suggestion that the Si IV and most of the C IV originate in the same place. The lower ionization absorption lines, Si III (1206.5 Å), Si II (1260.4 Å), S II(1259.4 Å), and N 1(1199.6 Å), are also measured. Effective widths of high- and low-ionization absorption lines are also correlated with each other. Such a correlation may indicate a real physical connection between hot and cold gas if the filling factor of the absorber is small and the number of such absorption regions in the line of sight is small. Physical properties of the hot gas- producing C IV and Si IV lines are also discussed. We suggest that there are about three such regions per kiloparsec.