Bullet Cluster

The Bullet cluster is the best current evidence for the nature of dark matter.

"The superb quality of the Chandra mirrors produced exquisite images of extended celestial X-ray sources, and this is certainly true for clusters of galaxies. The images revealed rich structure, associated with the process of formation and evolution of clusters - such as merging activity, or interaction of the central AGN with the intra-cluster medium. The availability of high-quality strong and weak lensing data allows detailed comparison of the gravitating mass against the properties of the X-ray emitting gas. In general, the total (dark + baryonic matter) masses inferred from weak lensing agree with those inferred from X-ray observations, but discrepancies betwen X-ray and strong lensing masses are commonly inferred in non-relaxed systems, which are presumably still forming, and are not in dynamical equilibrium. Particularly compelling results were inferred from the Chandra observations of the “bullet cluster” (1E0657-56; Fig. 2) by Markevitch et al. (2004) and Clowe et al. (2004). Those authors report that the cluster is undergoing a high-velocity ∼ 4500 km s −1 merger, evident from the spatial distribution of the hot, X-ray emitting gas, but this gas lags behind the subcluster galaxies. Furthermore, the dark matter clump, revealed by the weak-lensing map, is coincident with the collisionless galaxies, but lies ahead of the collisional gas. This – and other similar observations – allow good (and interesting) limits on the cross-section of the self-interaction of dark matter."