Richard Lieu, Dept. of Physics, University of Alabama in Huntsville
SSL Addition Conference Room at 3pm Tuesday December 18th
Since the mid 1990’s it has been increasingly apparent that material in and around clusters of galaxies is multi-phased, consisting of dark matter and hot, virialized X-ray emitting gas, and also cooler gases and possibly non-thermal particles. The early evidence for the latter two components was from EUV and soft X-ray observations from which it was inferred that a warm gas component lies predominantly at the outskirts of the cluster while relativistic particles may exist in the inner parts. With the advent of Chandra and XMM-Newton, the emphasis shifted to `gas fraction measurements’, as the mass ratio of X-ray to dark matter was found to be below the cosmic mean value inside clusters, increasing monotonically to outer radii.This points to an abundance of baryons `hiding’ in the outskirts, with sub-virial temperatures. A key question is whether the warm gas residing in filamentary structures between clusters and groups, as revealed by structure simulations, can account for the missing baryons. I will review these developments, as well as the more recent Sunyaev-Zel’dovich observations that provide evidence supporting the model of the radial distribution of sub-virialized and non-thermal particles in clusters and sheds new insights as to what they mean. I also present future prospects in this field with the launch (only 1-2 years away) of e-Rosita (the former SpectrumX Gamma Mission, now a German-Russian satellite). This mission has wide field imaging in both the ROSAT Carbon band and the EUVE Lex/B band.