A Dark Core in Abell 520

The rich cluster Abell 520 (z = 0.201) exhibits truly extreme and puzzling multiwavelength characteristics. It may best be described as a "cosmic train wreck." It is a major merger showing abundant evidence for ram pressure stripping, with a clear offset in the gas distribution compared to the galaxies (as in the Bullet Cluster, 1E 0657-558). However, the most striking feature is a massive dark core (721 h70 M☉/L☉B) in our weak-lensing mass reconstruction. The core coincides with the central X-ray emission peak, but is largely devoid of galaxies. An unusually low mass-to-light ratio region lies 500 kpc to the east, and coincides with a shock feature visible in radio observations of the cluster. Although a displacement between the X-ray gas and the galaxy/dark matter distributions may be expected in a merger, a mass peak without galaxies cannot be easily explained within the current collisionless dark matter paradigm. Interestingly, the integrated gas mass fraction (≈0.15), mass-to-light ratio (220 h70 M☉/L☉B), and position on the X-ray luminosity-temperature and mass-temperature relations are unremarkable. Thus, gross properties and scaling relations are not always useful indicators of the dynamical state of clusters.

[1]  N. Kaiser,et al.  Mapping the dark matter with weak gravitational lensing , 1993 .

[2]  Gravitational Lensing in Clusters of Galaxies: New Clues Regarding the Dynamics of Intracluster Gas , 1994, astro-ph/9405063.

[3]  T. Broadhurst,et al.  A Method for Weak Lensing Observations , 1994, astro-ph/9411005.

[4]  Galaxy cluster virial masses and Omega , 1995, astro-ph/9509034.

[5]  R. Carlberg,et al.  The CNOC Cluster Redshift Survey Catalogs. I. Observational Strategy and Data Reduction Techniques , 1996 .

[6]  S. White,et al.  A Universal Density Profile from Hierarchical Clustering , 1996, astro-ph/9611107.

[7]  H. Hoekstra,et al.  Weak Lensing Analysis of Cl 1358+62 Using Hubble Space Telescope Observations , 1998 .

[8]  H. Böhringer,et al.  The Mass Function of an X-Ray Flux-limited Sample of Galaxy Clusters , 1999, astro-ph/0111285.

[9]  Broos,et al.  Mitigating Charge Transfer Inefficiency in the Chandra X-Ray Observatory Advanced CCD Imaging Spectrometer. , 2000, The Astrophysical journal.

[10]  H. Hoekstra,et al.  Hubble Space Telescope Weak-Lensing Study of the z=0.83 Cluster MS 1054–03 , 1999, astro-ph/9910487.

[11]  C. Jones,et al.  A Textbook Example of a Bow Shock in the Merging Galaxy Cluster 1E0657-56 , 2001 .

[12]  M. Meneghetti,et al.  Giant cluster arcs as a constraint on the scattering cross-section of dark matter , 2000, astro-ph/0011405.

[13]  Observational Mass-to-Light Ratio of Galaxy Systems from Poor Groups to Rich Clusters , 2001, astro-ph/0112534.

[14]  Cosmological constraints from the X-ray gas mass fraction in relaxed lensing clusters observed with Chandra , 2002, astro-ph/0205007.

[15]  The Density Profile of Cluster-scale Dark Matter Halos , 2002, astro-ph/0206455.

[16]  A. Taylor,et al.  Probing the Distribution of Dark Matter in the A901/902 Supercluster with Weak Lensing , 2001, astro-ph/0111288.

[17]  S. Maddox,et al.  Weak Gravitational Lensing by a Sample of X-Ray Luminous Clusters of Galaxies. I. The Data Set , 2002 .

[18]  H. Dahle.  WEAK GRAVITATIONAL LENSING BY A SAMPLE OF X-RAY LUMINOUS CLUSTERS OF GALAXIES – III. SERENDIPITOUS WEAK LENSING DETECTIONS OF DARK AND LUMINOUS MASS CONCENTRATIONS. , 2003 .

[19]  U. Florida,et al.  Weak-Lensing Mass Reconstruction of the Interacting Cluster 1E 0657–558: Direct Evidence for the Existence of Dark Matter , 2003, astro-ph/0312273.

[20]  UCSD,et al.  Direct Constraints on the Dark Matter Self-Interaction Cross Section from the Merging Galaxy Cluster 1E 0657–56 , 2004 .

[21]  Henry C. Ferguson,et al.  The Evolution of the Optical and Near-Infrared Galaxy Luminosity Functions and Luminosity Densities to z ~ 2 , 2005 .

[22]  Moscow,et al.  Bow Shock and Radio Halo in the Merging Cluster A520 , 2004, astro-ph/0412451.

[23]  Mass profiles of galaxy cluster cores , 2005 .

[24]  C. Jones,et al.  ERRATUM: “CHANDRA SAMPLE OF NEARBY RELAXED GALAXY CLUSTERS: MASS, GAS FRACTION, AND MASS–TEMPERATURE RELATION” (2006, ApJ, 640, 691) , 2005, astro-ph/0507092.

[25]  J. Kneib,et al.  A Hubble Space Telescope lensing survey of X-ray luminous galaxy clusters – IV. Mass, structure and thermodynamics of cluster cores at z = 0.2 , 2005 .

[26]  The 2dF Galaxy Redshift Survey: Luminosity functions by density environment and galaxy type , 2005 .

[27]  Tucson,et al.  A Direct Empirical Proof of the Existence of Dark Matter , 2006, astro-ph/0608407.

[28]  B. Garilli,et al.  Accurate photometric redshifts for the CFHT legacy survey calibrated using the VIMOS VLT deep survey , 2006, astro-ph/0603217.

[29]  H. Hoekstra,et al.  The Shear Testing Programme – I. Weak lensing analysis of simulated ground-based observations , 2005, astro-ph/0506112.

[30]  A. Babul,et al.  The impact of mergers on relaxed X-ray clusters – I. Dynamical evolution and emergent transient structures , 2006, astro-ph/0608560.

[31]  How rare is the bullet cluster , 2006, astro-ph/0604443.

[32]  H. Hoekstra.  A comparison of weak-lensing masses and X-ray properties of galaxy clusters , 2007, 0705.0358.

[33]  Joint analysis of cluster observations. I. Mass profile of abell 478 from combined X-ray, sunyaev-zel'dovich, and weak-lensing data , 2007, astro-ph/0703372.

[34]  J. F. Navarro,et al.  Cosmic ménage à trois: the origin of satellite galaxies on extreme orbits , 2007, 0704.1773.