Numerical Simulations and the Observational Consequences of Merging Clusters of Galaxies

We present results from three-dimensional numerical simulations of head-on mergers between two clusters of galaxies using a hybrid Hydro/N-body code. In these simulations, the gaseous intracluster medium (ICM) is evolved self-consistently within a changing gravitational potential defined by the collisionless dark matter component. The ICM is represented by the equations of hydrodynamics which are solved by an Eulerian, finite-difference method. The cluster dark matter component is represented by the N-body particle distribution. A series of simulations have been conducted in which we have systematically varied the cluster-subcluster mass ratio between 8:1 and 1:1. We have also conducted one simulation in which the subcluster is essentially gas-free in order to show the relative importance of gas dynamics in the evolution of the merger remnant. We find that cluster-subcluster mergers can account for elongation of the X-ray morphology, isophotal twisting and centroid shifting as identified in the X-ray images of clusters. We also find an expansion of the X-ray core relative to the dark matter core which could explain recent results from gravitational lensing experiments. Mergers are found to result in patchy temperature morphologies like those observed in several nearby rich galaxy clusters. We find temperature inhomogeneities of several keV on linear scales smaller than 0.5 Mpc. Post-merger clusters exhibit non-equilibrium conditions (i.e., shocks, bulk flows, turbulence) for extended periods (2-3 Gyrs) after core passage. Such non-equilibrium conditions can translate into errors in cluster mass estimates of 25-50% which further translate into errors in the estimated baryon fraction in clusters. The gas dynamics resulting from mergers may play a role in the formation of various radio source morphologies including wide-angle tailed sources and cluster-wide radio halos. The sustained nature of the gas dynamics may extend the lifetimes of radio halos well beyond the canonical synchrotron lifetime of 10^8 years. The resulting inhomogeneities in the intracluster gas, such as shocks, may also play a role in the evolution of cluster galaxies. Individual galaxies interacting with the inhomogeneous ICM may experience an enhanced level of ram pressure which could induce star formation or abruptly truncate existing star formation.