  Several simple 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 kinetic energy, indicating
that streaming motions do not help to support the gas.
