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Shock waves driven in the intergalactic medium during the merging of galaxyclusters have been observed in X-ray imaging and spectroscopy. Fluid motionsinferred from the shock strength and morphology can be compared to the colddark matter (CDM) distribution inferred from gravitational lensing. A detailedreconstruction of the CDM kinematics, however, must take into account thenontrivial response of the fluid intracluster medium to the collisionless CDMmotions. We have carried out two-dimensional simulations of gas dynamics incluster collisions. We analyze the relative motion of the clusters, the bowshock wave, and the contact discontinuity and relate these to X-ray data. Wefocus on the "bullet cluster," 1E 0657-56, a near head-on collision ofunequal-mass clusters, for which the gas density and temperature jumps acrossthe prominent bow shock imply a high shock velocity 4,700 km/s. The velocity ofthe fluid shock has been widely interpreted as the relative velocity of the CDMcomponents. This need not be the case, however. An illustrative simulationfinds that the present relative velocity of the CDM halos is 16% lower thanthat of the shock. While this conclusion is sensitive to the detailed initialmass and gas density profile of the colliding clusters, such a decrease of theinferred halo relative velocity would increase the likelihood of finding 1E0657-56 in a LambdaCDM universe.Comment: 4 pages, 4 figure

The Cluster-Merger Shock in 1E 0657-56: Faster than a Speeding Bullet?