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Galaxy Cluster 1E0657-56
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Title: The Bullet Cluster revisited: New results from new constraints and improved strong lensing modelling technique
Authors: D. Paraficz, J.-P. Kneib, J. Richard, A. Morandi, M. Limousin, E. Jullo

We present a new detailed parametric strong lensing mass reconstruction of the Bullet Cluster (1E 0657-56) at z=0.296, based on new WFC3 and ACS HST imaging and VLT/FORS2 spectroscopy. The strong lensing constraints undergone deep revision, there are 14 (6 new and 8 previously known) multiply imaged systems, of which 3 have spectroscopically confirmed redshifts (including 2 newly measured). The reconstructed mass distribution includes explicitly for the first time the combination of 3 mass components: i) the intra-cluster gas mass derived from X-ray observation, ii) the cluster galaxies modelled by their Fundamental Plane (elliptical) and Tully-Fisher (spiral) scaling relations and iii) dark matter. The best model has an average rms value of 0.158" between the predicted and measured image positions for the 14 multiple images considered. The derived mass model confirms the spacial offset between the X-ray gas and dark matter peaks. The galaxy halos to total mass fraction is found to be f_s=115% for a total mass of 2.50.1 x10^14 solar mass within a 250 kpc radial aperture.

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RE: Dark Matter Discovery
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Title: Cluster Bulleticity
Authors: Richard Massey, Thomas Kitching, Daisuke Nagai

The unique properties of dark matter are revealed during collisions between clusters of galaxies, like the bullet cluster (1E 0657-56) and baby bullet (MACSJ0025-12). These systems provide evidence for an additional, invisible mass in the separation between the distribution of their total mass, measured via gravitational lensing, and their ordinary 'baryonic' matter, measured via its X-ray emission. Unfortunately, the information available from these systems is limited by their rarity. Constraints on the properties of dark matter, such as its interaction cross-section, are therefore restricted by uncertainties in the individual systems' impact velocity, impact parameter and orientation with respect to the line of sight.
Here we develop a complementary, statistical measurement in which every piece of substructure falling into every massive cluster is treated as a bullet. We define 'bulleticity' as the mean separation between dark matter and ordinary matter, and we measure a positive signal in hydrodynamical simulations. The phase space of substructure orbits also exhibits symmetries that provide a statistical null test. A real detection of bulleticity would provide evidence for a difference in the interaction cross-sections of baryonic and dark matter, and may rule out hypotheses of non-particulate dark matter that are otherwise able to model individual systems.

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Galaxy Cluster 1E0657-56
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Title: Dark Matter Annihilation Induced Gamma Ray Emission from Galaxy Cluster 1E0657-56
Authors: C. Zhang, G.-C. Liu

Based on minimal supersymmetric standard model, neutralino dark matter annihilation induced gamma ray emission from galaxy cluster 1E0657-56 is calculated. The merge of bullet-like subcluster with the main cluster is also investigated.

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GRavitational lEnsing Accuracy Testing 2008
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Are you good with statistics and interested in cosmology? Well, astronomers have issued a challenge to aid in the understanding of dark matter and dark energy the mysterious stuff that makes up roughly 95 percent of our universe. Thirty-eight astronomers from 19 international institutions are issuing this challenge, called the GRavitational lEnsing Accuracy Testing 2008

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Bullet Cluster
Credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.

This view of the Bullet Cluster, located about 3.8 billion light years from Earth, combines an image from NASA's Chandra X-ray Observatory with optical data from the Hubble Space Telescope and the Magellan telescope in Chile. This cluster, officially known as 1E 0657-56, was formed after the violent collision of two large clusters of galaxies. It has become an extremely popular object for astrophysical research, including studies of the properties of dark matter and the dynamics of million-degree gas.

In the latest research, the Bullet Cluster has been used to search for the presence of antimatter leftover from the very early Universe. Antimatter is made up of elementary particles that have the same masses as their corresponding matter counterparts - protons, neutrons and electrons - but the opposite charges and magnetic properties.

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1E 0657-56
Using data from NASA's Wilkinson Microwave Anisotropy Probe (WMAP), scientists have identified an unexpected motion in distant galaxy clusters. The cause, they suggest, is the gravitational attraction of matter that lies beyond the observable universe.

"The clusters show a small but measurable velocity that is independent of the universe's expansion and does not change as distances increase. We never expected to find anything like this" - lead researcher Alexander Kashlinsky at NASA's Goddard Space Flight Centre in Greenbelt, Md.

Kashlinsky calls this collective motion a "dark flow" in the vein of more familiar cosmological mysteries: dark energy and dark matter.

"The distribution of matter in the observed universe cannot account for this motion".

Hot X-ray-emitting gas in a galaxy cluster scatters photons from the cosmic microwave background. Clusters don't precisely follow the expansion of space, so the wavelengths of scattered photons change in a way that reflects each cluster's individual motion.
This results in a minute shift of the microwave background's temperature in the cluster's direction. Astronomers refer to this change as the kinematic Sunyaev-Zel'dovich (SZ) effect.

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1E 0657-56
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Title: The Cluster-Merger Shock in 1E 0657-56
Authors: Jun Koda, Milos Milosavljevic, Paul R. Shapiro, Daisuke Nagai, Ehud Nakar

The merging galaxy cluster 1E 0657-56, known as the "bullet cluster," is one of the hottest clusters known. The X-ray emitting plasma exhibits bow-shock-like temperature and density jumps. The segregation of this plasma from the peaks of the mass distribution determined by gravitational lensing has been interpreted as a direct proof of collisionless dark matter. If the high shock speed inferred from the shock jump conditions equals the relative speed of the merging CDM halos, however, this merger is predicted to be such a rare event in a LCDM universe that observing it presents a possible conflict with the LCDM model.
We examined this question using high resolution, 2D simulations of gas dynamics in cluster collisions to analyse the relative motion of the clusters, the bow shock, and the contact discontinuity, and relate these to the X-ray data for the bullet cluster. We find that the velocity of the fluid shock need not equal the relative velocity of the CDM components. An illustrative simulation finds that the present relative velocity of the CDM halos is 16% lower than that of the shock. While this conclusion is sensitive to the detailed initial mass and gas density profiles of the colliding clusters, such a decrease of the inferred halo relative velocity would significantly increase the likelihood of finding 1E 0657-56 in a LCDM universe.
(Conference proceedings based on a poster at Bash Symposium 2007)

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Title: Simulating the Bullet Cluster
Authors: Chiara Mastropietro, Andreas Burkert (USM, Munich)

We present high resolution N-body/SPH simulations of the interacting cluster 1E0657-56. The main and the sub-cluster are modelled using extended cuspy LCDM dark matter halos and isothermal beta-profiles for the collisional component. The hot gas is initially in hydrostatic equilibrium inside the global potential of the clusters. We investigate the X-ray morphology and derive the most likely impact parameters, mass ratios and initial relative velocities. We find that the observed displacement between the X-ray peaks and the associated mass distribution, the morphology of the bow shock, the surface brightness and projected temperature profiles across the shock discontinuity can be well reproduced by offset 1:6 encounters where the sub-cluster has initial velocity (in the rest frame of the main cluster) close to 2 times the virial velocity of the main cluster dark matter halo. A model with the same mass ratio and lower velocity (1.5 times the main cluster virial velocity) matches quite well most of the observations. However, it does not reproduce the morphology of the main cluster peak. Dynamical friction strongly affects the kinematics of the sub-cluster so that the low velocity bullet is actually bound to the main system at the end of the simulation. We find that a relatively high concentration (c=6) of the main cluster dark matter halo is necessary in order to prevent the disruption of the associated X-ray peak. For a selected sub-sample of runs we perform a detailed three dimensional analysis following the past, present and future evolution of the interacting systems. In particular, we investigate the kinematics of the gas and dark matter components as well as the changes in the density profiles and the motion of the system in the L_X-T diagram.

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The Bullet Cluster.kmz
Google Sky File

-- Edited by Blobrana at 16:46, 2008-01-12

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