  Shock waves driven in the intergalactic medium during the merging of galaxy
clusters have been observed in X-ray imaging and spectroscopy. Fluid motions
inferred from the shock strength and morphology can be compared to the cold
dark matter (CDM) distribution inferred from gravitational lensing. A detailed
reconstruction of the CDM kinematics, however, must take into account the
nontrivial response of the fluid intracluster medium to the collisionless CDM
motions. We have carried out two-dimensional simulations of gas dynamics in
cluster collisions. We analyze the relative motion of the clusters, the bow
shock wave, and the contact discontinuity and relate these to X-ray data. We
focus on the "bullet cluster," 1E 0657-56, a near head-on collision of
unequal-mass clusters, for which the gas density and temperature jumps across
the prominent bow shock imply a high shock velocity 4,700 km/s. The velocity of
the fluid shock has been widely interpreted as the relative velocity of the CDM
components. This need not be the case, however. An illustrative simulation
finds that the present relative velocity of the CDM halos is 16% lower than
that of the shock. While this conclusion is sensitive to the detailed initial
mass and gas density profile of the colliding clusters, such a decrease of the
inferred halo relative velocity would increase the likelihood of finding 1E
0657-56 in a LambdaCDM universe.
