Massive Coronae of Galaxies

There is reason to suspect that about half of the baryons are inpressure-supported plasma in the halos of normal galaxies, drawn in by gravityalong with about half of the dark matter. To be consistent with theobservations this baryonic component, the corona, would have to be hotter thanthe kinetic temperature of the dark matter in the halo so as to produceacceptable central electron densities. We ascribe this hotter plasmatemperature to the addition of entropy prior to and during assembly of thesystem, in an analogy to cluster formation. The plasma cooling time would belonger than the gravitational collapse time but, in the inner parts, shorterthan the Hubble time, making the corona thermally unstable to the formation ofa cloudy structure that may be in line with what is indicated by quasarabsorption line systems. The corona of an isolated spiral galaxy would be asource of soft X-ray and recombination radiation, adding to the more commonlydiscussed effects of stars and supernovae. In this picture the mass in thecorona is much larger than the mass in condensed baryons in a spiral galaxy.The corona thus would be a substantial reservoir of diffuse baryons that aresettling and adding to the mass in interstellar matter and stars, so that starformation in isolated spirals will continue well beyond the present epoch.Comment: 14 pages, 5 figure