AAS 207th Meeting, 8-12 January 2006
Session 139 Entropy, Convection and Cluster Properties
Poster, Wednesday, 9:20am-6:30pm, January 11, 2006, Exhibit Hall

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[139.02] The Distribution of Entropy in Groups and Clusters of Galaxies

E. J. Lloyd-Davies (University of Michigan)

In the simplest models of structure formation, involving gravitational collapse and shock heating, the properties of galaxy clusters are expected to follow simple scaling relations dependent only on system mass. Observations have long shown deviation from these scaling relations which to date have not been adequately explained by theoretical models. While some combination of cooling and heating of the gas is the likely culprit, the various proposed models fall short when it comes to reproducing observational details. A particularly useful diagnostic of these processes is the entropy of the intracluster gas, and particularly its distribution with radius. While a general feature of most models are isentropic cores that are larger in low mass systems, observations show excess entropy at all radii that can be observed.

We present an analysis of the ICM entropy distributions in a range of systems from the coolest groups up to the cluster regime using XMM-Newton and Chandra data. We use composite "blank sky" backgrounds matched to the instrumental and soft X-ray background of the target fields, allowing us to trace cool low surface brightness emission to large radii with greater confidence. Our sample is constructed to measure the entropy distributions out to large radii over a large range of system mass from the poor groups most affected by the processes under investigation up through the transition region between groups and clusters to rich systems where such processes have a small effect. The observed entropy distributions are compared with the theoretical predictions of various models in order to constrain the processes involved in cluster formation and evolution.

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