Numerical Simulations 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 as a massless fluid 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 find that cluster-subcluster mergers result in an elongation of both the cluster dark matter and gas distributions. The dark matter distribution is elongated parallel to the merger axis and accompanied by anisotropy in the dark matter velocity dispersion. Both the elongation and corresponding velocity anisotropy are sustained for more than 5 Gyr after the merger. The elongation of the gas distribution is also generally along the merger axis, although shocks and adiabatic compressions produce elongations perpendicular to the merger axis at various times during the merger. We also find a significant offset between dark matter and gas centroids in the period following core passage.

The gasdynamics is also severely affected by the cluster-subcluster merger. In these simulations, the subcluster enters the primary at supersonic speeds initiating bulk flows 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) radio sources. The most significant gasdynamics is seen to subside on timescales of 2 Gyr, although still significant dynamics is seen even after 5 Gyr. The merger-induced gasdynamics may also play a role in the formation of radio halo sources, and, consequently, the sustained nature of the gasdynamics may extend the lifetime of halos beyond the canonical synchrotron lifetime of the source.

Substructure, shocks, and adiabatic cooling during the merger can result in a very complex temperature structure within the intracluster medium. As a result of these mergers, we find temperature inhomogeneities of several keV on linear scales of <=0.5 Mpc.

Finally, these simulations indicate 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 for cluster mass estimates. The observable consequences of cluster mergers and their influence on cluster mass estimates are addressed in Roettiger, Burns, & Loken (1996).