A STUDY OF THE DARK CORE IN A520 WITH THE HUBBLE SPACE TELESCOPE: THE MYSTERY DEEPENS

We present a Hubble Space Telescope/Wide Field Planetary Camera 2 weak-lensing study of A520, where a previous analysis of ground-based data suggested the presence of a dark mass concentration. We map the complex mass structure in much greater detail, leveraging more than a factor of three increase in the number density of source galaxies available for lensing analysis. The "dark core" that is coincident with the X-ray gas peak, but not with any stellar luminosity peak, is now detected with more than 10σ significance. The ~1.5 Mpc filamentary structure elongated in the NE-SW direction is also clearly visible. Taken at face value, the comparison among the centroids of dark matter, intracluster medium, and galaxy luminosity is at odds with what has been observed in other merging clusters with a similar geometric configuration. To date, the most remarkable counterexample might be the Bullet Cluster, which shows a distinct bow-shock feature as in A520, but no significant weak-lensing mass concentration around the X-ray gas. With the most up-to-date data, we consider several possible explanations that might lead to the detection of this peculiar feature in A520. However, we conclude that none of these scenarios can be singled out yet as the definite explanation for this puzzle.

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

[2]  Dark Matter in MS 1224 from Distortion of Background Galaxies , 1994, astro-ph/9402017.

[3]  The Mass Distribution of the Most Luminous X-Ray Cluster RXJ 1347.5-1145 From Gravitational Lensing , 1996, astro-ph/9703189.

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

[5]  A. Kinney,et al.  Template ultraviolet to near-infrared spectra of star-forming galaxies and their application to K-corrections , 1996 .

[6]  Dynamical Mass Estimates of Large-Scale Filaments in Redshift Surveys , 1996, astro-ph/9605126.

[7]  A. S. Fruchter,et al.  Drizzle: A Method for the Linear Reconstruction of Undersampled Images , 1998 .

[8]  A. Lasenby,et al.  A maximum‐entropy method for reconstructing the projected mass distribution of gravitational lenses , 1998, astro-ph/9802159.

[9]  J. Ostriker,et al.  Observational Constraints on the Self-interacting Dark Matter Scenario and the Growth of Supermassive Black Holes , 2001, astro-ph/0108203.

[10]  J. Miralda-Escudé.  A Test of the Collisional Dark Matter Hypothesis from Cluster Lensing , 2000, astro-ph/0002050.

[11]  J. Kneib,et al.  Constraints on the Collisional Nature of the Dark Matter from Gravitational Lensing in the Cluster A2218 , 2002, astro-ph/0207045.

[12]  M. Giavalisco,et al.  The Great Observatories Origins Deep Survey: Initial results from optical and near-infrared imaging , 2003, astro-ph/0309105.

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

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

[15]  H. Hoekstra,et al.  Mass-to-Light Ratios of Galaxy Groups from Weak Lensing , 2005, astro-ph/0508328.

[16]  N. Benı́tez,et al.  Weak-Lensing Analysis of the z ≃ 0.8 Cluster CL 0152–1357 with the Advanced Camera for Surveys , 2004, astro-ph/0409304.

[17]  D. Coe,et al.  Hubble Space Telescope Advanced Camera for Surveys Weak-Lensing and Chandra X-Ray Studies of the High-Redshift Cluster MS 1054–0321 , 2005 .

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

[19]  Tucson,et al.  Strong and Weak Lensing United. III. Measuring the Mass Distribution of the Merging Galaxy Cluster 1ES 0657–558 , 2006, astro-ph/0608408.

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

[21]  Granada,et al.  Galaxies in the Hubble Ultra Deep Field. I. Detection, Multiband Photometry, Photometric Redshifts, and Morphology , 2006, astro-ph/0605262.

[22]  The speed of the ‘bullet’ in the merging galaxy cluster 1E0657−56 , 2007, astro-ph/0703232.

[23]  R. Bouwens,et al.  Discovery of a Ringlike Dark Matter Structure in the Core of the Galaxy Cluster Cl 0024+17 , 2007, 0705.2171.

[24]  H. Hoekstra,et al.  A Dark Core in Abell 520 , 2007, 0706.3048.

[25]  M. Sirianni,et al.  Principal Component Analysis of the Time- and Position-dependent Point-Spread Function of the Advanced Camera for Surveys , 2007, 0710.5560.

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

[27]  Edinburgh,et al.  Revealing the Properties of Dark Matter in the Merging Cluster MACS J0025.4–1222 , 2008, 0806.2320.

[28]  E. Ellingson,et al.  Cluster Abell 520: a perspective based on member galaxies. A cluster forming at the crossing of thre , 2008, 0809.3139.

[29]  J. Tyson,et al.  DARK MATTER IN THE GALAXY CLUSTER CL J1226+3332 AT z = 0.89 , 2008, 0810.0709.

[30]  Jonathan L. Feng,et al.  Halo-shape and relic-density exclusions of Sommerfeld-enhanced dark matter explanations of cosmic ray excesses. , 2009, Physical review letters.

[31]  David W. Harris,et al.  SCALING RELATIONS AND OVERABUNDANCE OF MASSIVE CLUSTERS AT z ≳ 1 FROM WEAK-LENSING STUDIES WITH THE HUBBLE SPACE TELESCOPE , 2011, 1105.3186.

[32]  J. Rhodes,et al.  Creation of cosmic structure in the complex galaxy cluster merger Abell 2744 , 2011, 1103.2772.

[33]  K. Kuijken,et al.  Mass, Light and Colour of the Cosmic Web in the Supercluster SCL2243-0935 (z=0.447) , 2011, 1102.4617.

[34]  L. Williams,et al.  Light/mass offsets in the lensing cluster Abell 3827: evidence for collisional dark matter? , 2011, 1102.3943.

[35]  D. Wittman,et al.  DISCOVERY OF A DISSOCIATIVE GALAXY CLUSTER MERGER WITH LARGE PHYSICAL SEPARATION , 2011, 1110.4391.

[36]  Sun Mi Chung,et al.  ON DARK PEAKS AND MISSING MASS: A WEAK-LENSING MASS RECONSTRUCTION OF THE MERGING CLUSTER SYSTEM A520, , 2012, 1209.2143.