Discovery of a ringlike dark matter structure in the core of the galaxy cluster Cl 0024+17

We present a comprehensive mass reconstruction of the rich galaxy cluster Cl0024+17 at z similar or equal to 0: 4 from ACS data, unifying both strong- and weak-lensing constraints. The weak-lensing signal from a dense distribution of background galaxies (similar to 120 arcmin (-2)) across the cluster enables the derivation of a high- resolution parameter-free mass map. The strongly lensed objects tightly constrain the mass structure of the cluster inner region on an absolute scale, breaking the mass-sheet degeneracy. The mass reconstruction of Cl0024+ 17 obtained in such a way is remarkable. It reveals a ringlike dark matter substructure at r similar to 75" surrounding a soft, dense core at r less than or similar to 50". We interpret this peculiar substructure as the result of a high-speed line-of-sight collision of two massive clusters similar to 1-2 Gyr ago. Such an event is also indicated by the cluster velocity distribution. Our numerical simulation with purely collisionless particles demonstrates that such density ripples can arise by radially expanding, decelerating particles that originally comprised the precollision cores. Cl 0024+ 17 can be likened to the bullet cluster 1E 0657 - 56, but viewed along the collision axis at a much later epoch. In addition, we show that the long-standing mass discrepancy for Cl 0024+ 17 between X-ray and lensing can be resolved by treating the cluster X-ray emission as coming from a superposition of two X-ray systems. The cluster's unusual X-ray surface brightness profile that requires a two isothermal sphere description supports this hypothesis.