Until recently, the situation concerning detection of line emission from hot gas has been murky. The tentative detection by Feldman et al. (1981) was not confirmed by Murthy et al. (1989). Martin and Bowyer (1990) present a spectrum for one of their UVX targets that contains a quite impressive and convincing CIV 1549 Å line, and a much less impressive O III] 1663 Å line. For their other targets, for some of which similar line emission was claimed, Martin and Bowyer present only tiny portions of their spectra, at the location of the claimed lines. The full spectra of all targets should be published.
The recent development on this topic is detection of very strong line emission in the Eridanus region by Murthy, Im, Henry, and Holberg (1993) using the Voyager spacecraft. Figure 2 shows the spectrum of their Target B, which shows strong emission lines of O VI 1032 / 1038 Å and C III 977 Å radiation. The lines are extremely strong, and predicted associated lines (see figure) should be detectable even with IUE. The region involved is one where there is a very strong soft X-ray enhancement (Burrows et al. 1993) and based on our other Voyager spectra is not typical of the general interstellar medium. What this suggests is that when a high-sensitivity sky survey (e.g., Kimble et al. 1990) is finally carried out, what will be revealed is a highly patchy structured hot interstellar medium.
Of course the highly structured character of the spectrum of Figure 2 shows that there is no possibility that the background at high latitudes is line emission. (To verify this, the reader should consult the individual spectra from the relevant references in Figure 1, rather than rely on Figure 1 itself.)
Figure 2. Spectrum of a region in Eridanus observed by Murthy, Im, Henry, and Holberg (1993). Strong solar system Lyman (1216 Å) has been subtracted. Emission lines of C III (977 Å) and O VI (1032/1038 Å) are seen. The solid line shows the emission that is expected (Hartigan et al. 1987) from a shock with a velocity of 180 km s-1, including two photon emission, plus appropriate dust-scattered light. The sensitivity of Voyager above 1200 Å is too low to allow detection of additional predicted lines of N V, C II, Si IV, O IV, and C IV, but those lines should be accessible to IUE and to the Hopkins Ultraviolet telescope.