Off‐Center Collisions between Clusters of Galaxies

We present numerical simulations of off-center collisions between galaxyclusters made using a new hydrodynamical code based on the piecewise-parabolicmethod (PPM) and an isolated multigrid Poisson solver. We have performed threegas-only high-resolution simulations of collisions between equal-mass clusterswith different values of the impact parameter (0, 5, and 10 times the coreradius). With these simulations we have studied the observational appearance ofthe merging clusters and the variation in equilibration time, luminosityenhancement during the collision, and structure of the merger remnant withvarying impact parameter. Observational evidence of an ongoing collision ispresent for 1-2 sound crossing times after the collision, but only for specialviewing angles. The remnant actually requires at least five crossing times toreach virial equilibrium. Since the sound crossing time can be as large as 1-2Gyr, the equilibration time can thus be a large fraction of the age of theuniverse. The final merger remnant is very similar for impact parameters ofzero and five core radii. It possesses a roughly isothermal core, with centraldensity and temperature twice the initial values. Outside the core thetemperature drops as r^-1, and the density roughly as r^-3.8. The core radiusshows a small increase due to shock heating during the merger. For an impactparameter of ten core radii the core of the remnant possesses a more flatteneddensity profile, with a steeper dropoff outside the core. In both off-centercases the merger remnant rotates, but only for the ten-core-radius case doesthis appear to have an effect on the structure of the remnant.