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Session 31 - The Diffuse Interstellar Medium, Progress and Puzzles - I.
Oral session, Tuesday, June 11
Historical Society,
In the early euphoria about the possible role of hot gas in galaxies, it was imagined that the disks and halos of large spiral galaxies ought to radiate much of their supernova power, about 10^42 ergs s-1, in X-rays. But when X-ray observations of such galaxies became possible they failed to detect emission at even a few percent of this level. With more sensitive telescopes, diffuse soft emission has been detected in nearby spirals, at a luminosity on the order of 10^39-10^40 ergs s-1. However, even this level is best considered an upper limit to the emission of interstellar hot gas, as other soft sources (stars, super soft sources, supernova remnants) may contribute appreciably. Modelers tend to find this emission curiously low. On the other hand, hot gas emission is readily found in galaxies with unusually high rates of star formation, with hot plumes or halos in starburst galaxies. Correlation between activity level and X-ray luminosity provides a better idea of what to expect from more modest galaxies, and therefore, useful constraints on models for the dissipation of the supernova power.
