Spectral Observations of Diffuse Far‐Ultraviolet Emission from the Hot Phase of the Interstellar Medium with the Diffuse Ultraviolet Experiment

One of the keys to interpreting the character and evolution of interstellarmatter 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 muchmore difficult to observe than the cooler high density components of the ISMbecause of its low density and lack of easily observable tracers. Because gasof this temperature emits mainly in the far ultraviolet (912 angstrom - 1800angstroms) and extreme ultraviolet (80 angstrom - 912 angstrom), and (for gashotter than 10^6 K) X-rays, observations in these bands can provide importantconstraints to the distribution of this gas. Because of interstellar opacity atEUV wavelengths, only FUV and X-ray observations can provide clues to theproperties of hot gas from distant regions. We present results from a searchfor FUV emission from the diffuse ISM conducted with an orbital FUVspectrometer, DUVE, which was launched in July, 1992. The DUVE spectrometer,which covers the band from 950 angstrom to 1080 angstrom with 3.2 angstromresolution, observed a region of low neutral hydrogen column density near thesouth galactic pole for a total effective integration time of 1583 seconds. Theonly emission line detected was a geocoronal hydrogen line at 1025 angstrom. Weare able to place upper limits to several expected emission features thatprovide constraints on interstellar plasma parameters. We are also able toplace limits on the continuum emission throughout the bandpass. We comparethese limits and other diffuse observations with several models of thestructure of the interstellar medium and discuss the ramifications.Comment: 23 pages, LaTeX, 10 eps figures, uses aaspp4.sty and Psfig/TeX Release 1.2, Minor editorial change