Discovery of a Ringlike Dark Matter Structure in the Core of the Galaxy

We present a comprehensive mass reconstruction of the rich galaxy cluster Cl 0024+17 at z ≃ 0.4 from ACS data, unifying both strongand weak-lensing constraints. The weak-lensing signal from a dense distribution of background galaxies (∼ 120 arcmin) 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 Cl 0024+17 obtained in such a way is remarkable. It reveals a ringlike dark matter substructure at r ∼ 75 surrounding a soft, dense core at r . 50. We interpret this peculiar sub-structure as the result of a high-speed line-of-sight collision of two massive clusters ∼ 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 pre-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, 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 Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218. UCO/Lick Observatory, University of California, Santa Cruz, CA 95064. STScI, 3700 San Martin Drive, Baltimore, MD 21218. School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel. Inst. Astrof́ısica de Andalućıa (CSIC), Camino Bajo de Huétor, 24, Granada 18008, Spain Department of Physics and Astronomy, Washington State University, Pullman, WA 99164