Hubble Finds Ring of Dark Matter The Astrophysical Journal, 661: , 2007 June 1

About the Object Object Name: Cl Object Description: Galaxy Cluster Position (J2000): R.A. 00h 26m 35s Dec. +17° 09' 43" Constellation: Pisces Distance: Redshift z = (1.5 billion parsecs) Instrument: ACS/WFC Exposure Date(s): November 2004 Exposure Time: 14.5 hours About the Data

The distant galaxies appear distorted because their light is being bent and magnified by the gravity of Cl , an effect called gravitational lensing.

A nonparametric mass reconstruction method combing strong- and weak-lensing signal The weak-lensing signal from a dense distribution of background galaxies (~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.

A1 A2 A3 A5 A4 The well-known five multiple images at z = are labeled as A1~A5.

Source image reconstruction of the five well- known multiple images at z = The delensed image of each arc predicted from the deflection potential. Note that the orientation, parity, and size of these delensed images are consistent with each other.

Mass reconstruction of Cl

The ring-like structure is evident in the blue map of the cluster's dark matter distribution. The map is superimposed on a Hubble image of the cluster.

Interpretation The research suggested the cluster had collided with another cluster 1 to 2 billion years ago. (published in 2002 by Oliver Czoske of the Argeleander-Institut fur Astronomie at the Universitat Bonn) Quite a few numerical simulations have shown that the ringlike structure can arise from a radial density propagation in a high speed collision of two galaxies.

Side view of cluster collision Cluster collision as seen from Earth This peculiar substructure can be interpreted as the result of a high-speed line-of-sight collision of two massive clusters 1~2 Gyr ago.

Conclusion This is the first time to detect a dark matter distribution that differs from the distribution of both the galaxies and the hot gas. The ring's discovery is among the strongest evidence yet that dark matter exists.