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

TL;DR

This paper reveals a ringlike dark matter structure in galaxy cluster Cl 0024+17, interpreted as a collision remnant, using combined strong and weak gravitational lensing data to achieve a high-resolution mass map.

Contribution

It provides the first detailed mass reconstruction showing a ringlike dark matter substructure, suggesting a recent high-speed collision of galaxy clusters as its origin.

Findings

Discovery of a ringlike dark matter structure at ~75" radius

Reconciliation of X-ray and lensing mass discrepancies

Simulation supporting collision-induced density ripples

Abstract

We present a comprehensive mass reconstruction of the rich galaxy cluster Cl 0024+17 at z~0.4 from ACS data, unifying both strong- and weak-lensing constraints. The weak-lensing signal from a dense distribution of background galaxies (~120 per square 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 X-ray surface brightness profile that requires a two isothermal sphere description supports this hypothesis.