Off‐Axis Cluster Mergers: Effects of a Strongly Peaked Dark Matter Profile

Abstract

We present a parameter study of offset mergers between clusters of galaxies. Using the Eulerian hydrodynamics/ N -body code COSMOS, we simulate mergers between nonisothermal, hydrostatic clusters with a steep central dark matter density profile and a β-model gas profile. We constrain global properties of the model clusters using observed cluster statistical relationships. We consider impact parameters between 0 and 5 times the dark matter scale radius and mass ratios of 1 : 1 and 1 : 3. The morphological changes, relative velocities, and temperature jumps we observe agree with previous studies using the King profile for the dark matter. We observe a larger jump in X-ray luminosity (~4-10 times) than in previous work, and we argue that this increase is most likely a lower limit due to our spatial resolution. We emphasize that luminosity and temperature jumps due to mergers may have an important bearing on constraints on Ω derived from the observation of hot clusters at high redshift. Shocks are relatively weak in the cluster cores; hence, they do not significantly increase the entropy there. Instead, shocks create entropy in the outer regions, and this high-entropy gas is mixed with the core gas during later stages of the merger. Ram pressure initiates mixing by displacing the core gas from its potential center, causing it to become convectively unstable. The resulting convective plumes produce large-scale turbulent motions with eddy sizes up to several hundred kiloparsecs. This turbulence is pumped by dark matter-driven oscillations in the gravitational potential. Even after nearly a Hubble time these motions persist as subsonic turbulence in the cluster cores, providing 5%-10% of the support against gravity. The dark matter oscillations are also reflected in the extremely long time following a merger required for the remnant to reach virial equilibrium.