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Final Activity Report Summary - ISBGCXCGWP (Investigating similarity-breaking in galaxy clusters with XMM-Newton and Chandra)

The two main objectives of the project were to i) test the current paradigm of dark matter-driven hierarchical gravitational collapse, and ii) to examine the question of how non-gravitational processes affect the observed properties of the X-ray emitting intracluster medium. These tests were performed using X-ray observations of galaxy clusters, the largest virialised structures in the Universe.

Clusters contain 80% or more dark matter, which cannot be seen. Only 5% of the mass lies in the galaxies: the rest of the mass lies in the intracluster medium which is heated to temperatures of millions of degrees and which emits X-rays. Supposing that the gas is at rest in the potential well of the dark matter, we can calculate the mass profile of the cluster from spatially resolved X-ray observations. Once scaled appropriately, these mass profiles were all similar, showing that clusters covering more than an order of magnitude in mass are structurally similar. Moreover, it was shown that the properties of the mass profiles were in excellent agreement with the predictions from numerical simulations, further supporting the current model of gravitational collapse.

Making simple assumptions about the X-ray gas in galaxy clusters it is possible to show that relations exist between the global X-ray properties, such as the X-ray temperature or luminosity, and the mass and redshift. These relations are known as scaling laws. It has been known for some time that there is disagreement between the observed scaling laws and those predicted by structure formation through gravitational collapse alone, leading to the widespread belief that non-gravitational processes are affecting the gas properties. The most fundamental parameter in investigations of this kind is the entropy of the gas. In this project, we showed that the entropy of the gas in poor clusters was higher, relative to that expected in gravitational collapse, than that in rich clusters. At the same time it was shown that the entropy structure was similar, outside the very central regions. These observations are crucial for distinguishing between the different candidate non-gravitational processes.

The most important advances made in this project included: i) the discovery of a universal mass profile, showing observationally for the first time that the underlying dark matter distribution in clusters is self-similar; ii) the establishment of a benchmark mass-temperature relation for observations and simulations (since observationally confirmed by another group); iii) the discovery that cluster entropy profiles are self-similar outside the core regions, barring a normalisation factor which depends on the mass of the system, giving strong constraints on the nature of the non-gravitational processes affecting the X-ray properties of clusters; iv) investigation of systematic differences between X-ray and lensing mass estimates; v) investigation of the dynamics of interacting clusters.

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