Head-on mergers of systems containing gas

Several simple head-on mergers between model galaxy clusters containing a mixture of gas and dark matter are examined, testing the coupling of the gas to the underlying collisionless material. The gas is shocked, irreversibly dissipating the energy fed into it by the collisionless 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 central gas has little residual disordered kinetic energy, indicating that small-scale streaming motions do not help to support the gas.