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Session 31 - The Diffuse Interstellar Medium, Progress and Puzzles - I.
Oral session, Tuesday, June 11
Historical Society,
The immediate solar neighborhood, the nearest 100 pc or so, is filled with hot low-density gas that has a substantial thermal pressure and is a dominant factor in the energy budget of the local interstellar medium. This gas produces substantial soft thermal soft x-ray emission below 1/4 keV, but is difficult to observe outside the soft x-ray band. Sounding rocket and satellite all-sky surveys in several broad x-ray energy bands provided the general picture of this million-degree gas surrounding the solar system, extending perhaps as far as 100 pc in the galactic plane and roughly twice that far out of the plane, and showing no signs of being absorbed by intervening neutral gas. ROSAT observations of "shadows" in the diffuse x-ray background have subsequently determined the spatial locations of this hot gas along a few lines of sight: closer than 65 pc in some low galactic latitude directions, farther than several hundred pc in some high galactic latitude directions. Other large regions of hot gas are seen nearby, within a few hundred parsecs, but the galactic filling factor of the hot gas is unknown. DXS obsrvations of the spectra of the diffuse soft x-ray background confirm that the emission in the plane is thermal, but the spectrum of the hot gas is not fit by standard thermal equilibrium models. The temperature distribution, emission measure, ionization distributions, and metallicity of the hot gas are unknown. This talk emphasizes a few of the things that are clear about the hot ISM, and discusses a few of the most puzzling problems in our understanding of the nature of the diffuse hot gas. New instruments with hig spectral resolution will make it possible to address some of these problems.
