Spectral Observations of Diffuse FUV Emission from the Hot Phase of the Interstellar Medium with DUVE ( the Diffuse Ultraviolet Experiment )

One of the keys to interpreting the character and evolution of interstellar matter in the galaxy is understanding the distribution of the low density hot (10 5 K − 10 6 K) phase of the interstellar medium (ISM). This phase is much more difficult to observe than the cooler high density components of the ISM because of its low density and lack of easily observable tracers. Because gas of this temperature emits mainly in the far ultraviolet (912Å-1800Å) and extreme ultraviolet (80Å-912Å), and (for gas hotter than 10 6 K) X-rays, observations in these bands can provide important constraints to the distribution of this gas. Because of interstellar opacity at EUV wavelengths, only FUV and X-ray observations can provide clues to the properties of hot gas from distant regions. We present results from a search for FUV emission from the diffuse ISM conducted with an orbital FUV spectrometer, DUVE, which was launched in July, 1992. The DUVE spectrometer, which covers the band from 950Å to 1080Å with 3.2 ˚ A resolution, observed a region of low neutral hydrogen column density near the south galactic pole for a total effective integration time of 1583 seconds. The only emission line detected was a geocoronal hydrogen line at 1025Å. We are able to place upper limits to several expected emission features that provide constraints on interstellar plasma parameters. We are also able to place limits on the continuum emission throughout the bandpass. We compare these limits and other diffuse observations with several models of the structure of the interstellar medium and discuss the ramifications.