The Physics and Observational Signatures of Galaxy Cluster Mergers

Chatzikos, Marios, Department of Astronomy, University of Virginia
Sarazin, Craig, Department of Astronomy, University of Virginia

The structure of galaxy clusters has long been thought to scale with the system mass. However, recent X-ray observations by Chandra and XMM-Newton have revealed that cluster structure depends weakly on the mass. In this thesis, we study whether the departure from scale-free structure introduces a mass-dependence to the growth of clusters by collisions with similar systems, called mergers. We have compiled a library of 156 simulations of binary cluster mergers, the Simulation Library of Astrophysical galaxy cluster Mergers (SLAM) database, that covers an unprecedented parameter space volume. The structures of our initial models are in excellent agreement with recent X-ray observations. We utilize one million particles to resolve the more massive of the clusters, each composed of dark matter and gas. We conduct our simulations with the TreeSPH simulation code Gadget-2. Our simulations are fully converged. We compare the evolution of mergers across the parameter space covered by our simulations and find evidence that the merging process is not self-similar, due to differences in the initial cluster structures. All remnants experience mass loss that depends on the merger configuration, and has a different dependence on the initial mass ratio for the gas and dark matter. Equal-mass mergers lead to substantial dark matter loss, due to violent relaxation. Gas mass loss is more significant for high mass ratios, due to ram pressure variability during the merger. The properties of the gas core depend sensitively on the initial merger parameters. However, beyond the core the cluster structure is independent of the remnant's merger history. We quantify substructure in X-ray images, and find that it is not intimately linked to the dynamical state of the cluster. However, substructure may survive in X-ray images for up to 5 Gyr complicating the interpretation of X-ray images. We conclude that substructure seen in low-redshift images of X-ray faint clusters may be due to high-redshift mergers.

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PHD (Doctor of Philosophy)
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