The Cluster-Merger Shock in 1E 0657-56
  The merging galaxy cluster 1E 0657-56, known as the "bullet cluster," is one of the hottest clusters known. The X-ray emitting plasma exhibits bow-shock-like temperature and density jumps. The segregation of this plasma from the peaks of the mass distribution determined by gravitational lensing has been interpreted as a direct proof of collisionless dark matter. If the high shock speed inferred from the shock jump conditions equals the relative speed of the merging CDM halos, however, this merger is predicted to be such a rare event in a LCDM universe that observing it presents a possible conflict with the LCDM model.   We examined this question using high resolution, 2D simulations of gas dynamics in cluster collisions to analyze the relative motion of the clusters, the bow shock, and the contact discontinuity, and relate these to the X-ray data for the bullet cluster. We find that the velocity of the fluid shock need not equal the relative velocity of the CDM components. 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 profiles of the colliding clusters, such a decrease of the inferred halo relative velocity would significantly increase the likelihood of finding 1E 0657-56 in a LCDM universe.   (Conference proceedings based on a poster at Bash Symposium 2007) 
Jun Koda, Milos Milosavljevic, Paul R. Shapiro, Daisuke Nagai, Ehud   Nakar