Discovery of a Dark Matter Ring in CL Myungkook James Jee Johns Hopkins University, Baltimore, USA July 3, 2007 IAP Myungkook James Jee Johns Hopkins University, Baltimore, USA July 3, 2007 IAP

Presentation Outline 1.Introduction  A brief history  Why is CL0024 interesting? 2.Gravitational Lensing  Problems with previous analyses  A new technique 3.Results and Interpretation 4.Frequently Asked Questions 5.Conclusions 1.Introduction  A brief history  Why is CL0024 interesting? 2.Gravitational Lensing  Problems with previous analyses  A new technique 3.Results and Interpretation 4.Frequently Asked Questions 5.Conclusions

A Brief History  Discovered by Humason & Sandage (1957).  One of the first targets showing the Butcher-Omler effect (Dressler & Gunn 1982).  Gravitational arcs were discovered by Koo (1988).  Depletion effect were detected by Fort et al. (1997).  Comprehensive mass reconstruction with HST/WFPC2 (Tyson et al. 1998).  Spectroscopic redshift of the arcs was obtained by Broadhurst et al. (2000).  Wide-field mass reconstruction (Kneib et al. 2003).  Discovered by Humason & Sandage (1957).  One of the first targets showing the Butcher-Omler effect (Dressler & Gunn 1982).  Gravitational arcs were discovered by Koo (1988).  Depletion effect were detected by Fort et al. (1997).  Comprehensive mass reconstruction with HST/WFPC2 (Tyson et al. 1998).  Spectroscopic redshift of the arcs was obtained by Broadhurst et al. (2000).  Wide-field mass reconstruction (Kneib et al. 2003).

Why is CL0024 so interesting?  The cluster has the five multiple images that possess clear substructures.  A factor of 2-4 difference between X-ray and lensing masses (Ota et al. 2004; Zhang et al. 2005).  Spectroscopic data suggest that there was a high-speed line-of sight collision between two sub-clusters (Czoske et al. 2002).  The cluster has the five multiple images that possess clear substructures.  A factor of 2-4 difference between X-ray and lensing masses (Ota et al. 2004; Zhang et al. 2005).  Spectroscopic data suggest that there was a high-speed line-of sight collision between two sub-clusters (Czoske et al. 2002).

Solution is not unique! The location of the critical curve does not change under the above transformation.

Mass-Sheet Degeneracy A family of solutions predicting the five multiple images! This also applies to weak-lensing analysis. Assumptions are required to break the degeneracy. Cause of discrepancy between strong- and weak-lensing.

Multiple systems at different redshifts. However… Multiple image systems are found astride critical curves. They are only sensitive to the inner mass profile. The entire mass profile is obtained by extrapolating the inner profile.

Shapes of Background Galaxies They contain tremendous information on the mass distribution in the cluster outskirts.

Weak-lensing Mass Reconstructions CL 0152 at z=0.84MS 1054 at z=0.83Abell 2218 at z=0.17 CL 1252 at z=1.2 Lynx clusters at z=1.3

Our Mass Reconstruction Algorithm <- The target function we minimize. <- Strong-lensing constraints.

Our Mass Reconstruction Algorithm <- The target function we minimize. <- Strong-lensing constraints. <- Weak-lensing constraints (ellipticity).

Our Mass Reconstruction Algorithm <- The target function we minimize. <- Strong-lensing constraints. <- Weak-lensing constraints (ellipticity). <- Entropy-regularization.

Presentation Outline 1.Introduction  A brief history  Why is CL0024 interesting? 2.Gravitational Lensing 3.Results and Interpretation 4.Frequently Asked Questions 5.Conclusions 1.Introduction  A brief history  Why is CL0024 interesting? 2.Gravitational Lensing 3.Results and Interpretation 4.Frequently Asked Questions 5.Conclusions

Mass Reconstruction of CL0024 OriginalBackground subtracted Discovery of a dark matter ring at 75”

Possible Causes of the Ring?  Image alignment errors  Inaccurate geometric distortion correction  Imperfect PSF correction  Inconsistency between Strong- and Weak-lensing signals  Regularization artifacts  Edge effect (finite field inversion)  Data overfitting  Sampling violation  Image alignment errors  Inaccurate geometric distortion correction  Imperfect PSF correction  Inconsistency between Strong- and Weak-lensing signals  Regularization artifacts  Edge effect (finite field inversion)  Data overfitting  Sampling violation

Radial Profile and Tangential Shear The ring though a low-contrast structure appears as a bump at 75”. The tangential shear also reflects the peculiar radial mass profile. The error bars are overestimation of the true statistical errors because any deviation from the azimuthal symmetry contribute to the error bars.

We are not alone in the detection of the dip in the tangential shear! Kneib et al. (2003)

Mass vs Galaxies In the cluster core, bright elliptical galaxies define mass peaks. The ring is not well traced by the cluster galaxies.

Origin of the Ring Czoske et al. (2002) A high-speed collision?

Numerical Simulation of Cluster Collision Initially, the two clusters contract because of the increased gravity. About 0.5 Gyrs after the core Impact, the dark matter particles flow outward. At 1 Gyrs since the impact, the Decelerated outflow creates shell-like structures, which appear as a ring when projected onto the plane of the sky.

Presentation Outline 1.Introduction 2.Gravitational Lensing  Problems with previous analyses  A new technique 3.Results and Interpretation 4.Frequently Asked Questions 5.Conclusions 1.Introduction 2.Gravitational Lensing  Problems with previous analyses  A new technique 3.Results and Interpretation 4.Frequently Asked Questions 5.Conclusions

FAQ 1. Why can’t we see the ring in galaxy distribution? 1.Galaxies sample the underlying dark matter only sparsely. 2.The ring is a low-contrast structure. 1.Galaxies sample the underlying dark matter only sparsely. 2.The ring is a low-contrast structure.

FAQ 1. Why can’t we see the ring in galaxy distribution?- Continued.

FAQ 2. Can’t the dip at 75” in the tangential shear profile be caused by an isolated mass clump instead of the ring? 1.The algorithm has been tested for artificial clusters with/without a ring. 2.There seems to be no isolated mass clumps at 75” in CL The algorithm has been tested for artificial clusters with/without a ring. 2.There seems to be no isolated mass clumps at 75” in CL

FAQ 2. Can’t the dip at 75” in the tangential shear profile be caused by an isolated mass clump instead of the ring? 3. The secondary mass clump in Abell 2218 is detected as an isolated mass clump in the mass reconstruction. Jee et al., in prep.

FAQ 3. Galaxy Clusters are dynamically hot. Then why the ring? It must be a high-speed collision (~3000 km/s) to create shells (rings). However, orbits of dark matter particles are important. It must be a high-speed collision (~3000 km/s) to create shells (rings). However, orbits of dark matter particles are important.

Highly Tangential

Moderately Tangential

Isotropic

FAQ 4. Why doesn’t the ICM of the cluster follow the ring?  When the system is fully relaxed, the gas of the cluster should follow the potential well dominated by the cluster dark matter.

FAQ 4. Why doesn’t the ICM of the cluster follow the ring? Markevitch 2007, private communication “I have to admit that the dark matter ring, as presented in the paper, looks like some kind of an artifact. However, there is an additional piece of data that, to me, adds confidence in the reality of this result (or at least shows that something strange is certainly going on in this cluster). If you play with colors in the Chandra image of this cluster, you will notice at least 2 rings, or edges, of X-ray emission around the center, one around r=40", and another right around r=75". You've probably missed them because you smoothed the image too heavily.”

Conclusions  We discovered a ringlike dark matter structure in CL  We claim that the ring is the result of the high- speed collision occurred along the line-of-sight direction.  The Chandra data suggest that the dark matter ring may be also traced by the cluster gas.  Useful laboratory where many fundamental questions in physics can be studied.  We discovered a ringlike dark matter structure in CL  We claim that the ring is the result of the high- speed collision occurred along the line-of-sight direction.  The Chandra data suggest that the dark matter ring may be also traced by the cluster gas.  Useful laboratory where many fundamental questions in physics can be studied.

Resolving the Mass Discrepancy The X-ray surface brightness profile is peculiar (open circles). One component model (dashed) cannot explain the observed profile. Two component model (solid) can nicely explain the peculiarity. Jee et al. (2007)