HEAD-ON MERGERS OF SYSTEMS CONTAINING GAS

Several simple head-on mergers between model galaxy clusters containin g a mixture of gas and dark matter are examined, testing the coupling of the gas to the underlying collisionless material. The gas is shocke d, irreversibly dissipating the energy fed into it by the collisionles s component, and forms a resolved constant-density core. For the dark matter, however, admixture of phase-space vacuum is not very efficient and a constant-density core is not produced. In the final state the c entral gas has little residual disordered kinetic energy, indicating t hat small-scale streaming motions do not help to support the gas.