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R. Lieu (Physics Dept., Univ. of Alabama at Huntsville)
Seven years since its discovery, with confirmation in the form of `DNA fingerprint' evidence provided by XMM Newton observations, we are now at a stage where the following statements can be made for certain about soft excess emission from clusters of galaxies. (a) The phenomenon is real; for many clusters it is present at all radii, and exhibits a wide variety of spectral and spatial behavior among objects. (b) For the outer parts of five clusters, the detection of bright O VII & O VIII emission lines at the cluster redshift is a major breakthrough, as it proves that at least a good fraction of the soft excess is caused by a warm and massive component of baryonic matter (i.e. our original interpretation). (c) For the inner regions the responsible emission process is unclear, here the soft excess is very bright, and remains evident despite the luminosity enhancement due to `cooling flow', rekindling the possibility of a non-thermal population with a pressure that scales (increases) with that of the cooling virial gas. I review the above properties, and explain how the current idea of invoking the warm component as projection filaments lying principally beyond a cluster can avoid many of the earlier fundamental difficulties. Nonetheless, severe constraints on the filament model are imposed by opacity effects, the observed emission radial profile, and the total baryonic mass budget of a cluster (at present , predictions from the warm/hot intergalactic medium code falls short of the soft excess luminosity by several orders of magnitude). Future prospects of this important new field are very exciting indeed. While XMM charts the nearby clusters, BEST and Constellation X will be able to investigate clusters at higher redshifts where the warm component plays an even more prominent role. In parallel, the increasigly sophisticated Sunyaev-Zeldovich measurements provide a different view of this aspect of cluster evolution, and offers a means of clinching the non-thermal constribution to the soft excess.
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The author(s) of this abstract have provided an email address for comments about the abstract: lieur@cspar.uah.edu
Bulletin of the American Astronomical Society,
35#2
© 2003. The American Astronomical Soceity.