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We present a comprehensive mass reconstruction of the rich galaxy cluster Cl0024+17 at z~0.4 from ACS data, unifying both strong- and weak-lensingconstraints. The weak-lensing signal from a dense distribution of backgroundgalaxies (~120 per square arcmin) across the cluster enables the derivation ofa high-resolution parameter-free mass map. The strongly-lensed objects tightlyconstrain the mass structure of the cluster inner region on an absolute scale,breaking the mass-sheet degeneracy. The mass reconstruction of Cl 0024+17obtained in such a way is remarkable. It reveals a ringlike dark mattersubstructure at r~75" surrounding a soft, dense core at r~50". We interpretthis peculiar sub-structure as the result of a high-speed line-of-sightcollision of two massive clusters 1-2 Gyr ago. Such an event is also indicatedby the cluster velocity distribution. Our numerical simulation with purelycollisionless particles demonstrates that such density ripples can arise byradially expanding, decelerating particles that originally comprised thepre-collision cores. Cl 0024+17 can be likened to the bullet cluster 1E0657-56,but viewed $along$ the collision axis at a much later epoch. In addition, weshow that the long-standing mass discrepancy for Cl 0024+17 between X-ray andlensing can be resolved by treating the cluster X-ray emission as coming from asuperposition of two X-ray systems. The cluster's unusual X-ray surfacebrightness profile that requires a two isothermal sphere description supportsthis hypothesis.

Discovery of a Ringlike Dark Matter Structure in the Core of the Galaxy Cluster Cl 0024+17