NUMERICAL SIMULATIONS OF MERGING CLUSTERS OF GALAXIES

We present results from three-dimensional numerical simulations of hea d-on mergers between two clusters of galaxies using a hybrid hydro/N-b ody code. In these simulations, the gaseous intracluster medium (ICM) is evolved as a massless fluid within a changing gravitational potenti al defined by the collisionless dark matter component. The ICM is repr esented by the equations of hydrodynamics which are solved by an Euler ian, finite-difference method. The cluster dark matter component is re presented by the N-body particle distribution. A series of simulations have been conducted in which we have systematically varied the cluste r-subcluster mass ratio between 8:1 and 1:1. We find that cluster-subc luster mergers result in an elongation of both the cluster dark matter and gas distributions. The dark matter distribution is elongated para llel to the merger axis and accompanied by anisotropy in the dark matt er velocity dispersion. Both the elongation and corresponding velocity anisotropy are sustained for more than 5 Gyr after the merger. The el ongation of the gas distribution is also generally along the merger ax is, although shocks and adiabatic compressions produce elongations per pendicular to the merger axis at various times during the merger. We a lso find a significant offset between dark matter and gas centroids in the period following core passage. The gasdynamics is also severely a ffected by the cluster-subcluster merger. In these simulations, the su bcluster enters the primary at supersonic speeds initiating bulk hows that can exceed 2000 km s(-1). The width of the bulk flows are seen to range between several hundred kiloparsecs to nearly 1 Mpc. We believe the bulk flows can produce the bending of wide-angle tailed (WAT) rad io sources. The most significant gasdynamics is seen to subside on tim escales of 2 Gyr, although still significant dynamics is seen even aft er 5 Gyr. The merger-induced gasdynamics may also play a role in the f ormation of radio halo sources, and, consequently, the sustained natur e of the gasdynamics may extend the lifetime of halos beyond the canon ical synchrotron lifetime of the source. Substructure, shocks, and adi abatic cooling during the merger can result in a very complex temperat ure structure within the intracluster medium. As a result of these mer gers, we find temperature inhomogeneities of several keV on linear sca les of less than or equal to 0.5 Mpc. Finally, these simulations indic ate that even relatively high mass-ratio mergers (e.g., 8:1) result in nonequilibrium conditions for an extended period of time. The period of time with the most significant dynamical evolution is within 2 Gyr after core passage. The nonequilibrium conditions have implications fo r cluster mass estimates. The observable consequences of cluster merge rs and their influence on cluster mass estimates are addressed in Roet tiger, Burns, & Loken (1996).