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Bullet Cluster
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Title: Mapping the dark trajectory of the Bullet Cluster
Authors: Xiao-Jun Bi, HongSheng Zhao

We model the electrons/positrons produced by dark matter annihilations in the colliding galaxy cluster system 1E0561 (Bullet). These, confined by the Magnetic filed, mark a clear track of the bullet, which passes through the main cluster with a speed of 3000-5000 km/s. Adding the effect of subhalos in each cluster we find the annihilation rate is enhanced greatly and the density of positrons in the trail is similar to that within the bullet cluster. These open the possibility of a unambiguous detection of the annihilation signal through, e.g., SZ effects, at far away from the thermal electrons.

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RE: Dark Matter Discovery
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The orbiting X-ray telescopes XXM-Newton and Chandra have caught a pair of galaxy clusters merging into a giant cluster. The discovery adds to existing evidence that galaxy clusters can collide faster than previously thought.
When individual galaxies collide and spiral into one another, they discard trails of hot gas that stretch across space, providing signposts to the mayhem. Recognising the signs of collisions between whole clusters of galaxies, however, is not as easy.
When individual galaxies collide and spiral into one another, they discard trails of hot gas that stretch across space, providing signposts to the mayhem. Recognising the signs of collisions between whole clusters of galaxies, however, is not as easy.


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Posts: 131433
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The Bullet Cluster
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HI-RES MOV
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This animation shows an artist's representation of the huge collision in the Bullet Cluster.

From here on Earth, we are able to see the Bullet Cluster across our line of sight, and hence the process of merger can be seen clearly. In Abell 576, where two galaxy clusters are merging head on, in our line of sight, it is harder to disentangle.
During the collision the hot gas (shown in red) in each cluster is slowed and distorted by a drag force, similar to air resistance. A bullet-shaped cloud of gas forms in one of the clusters.

Credits: NASA/CXC/M.Weiss


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RE: Dark Matter Discovery
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Title: A New Force in the Dark Sector?
Authors: Glennys R. Farrar, Rachel A. Rosen

We study the kinematics of dark matter using the massive cluster of galaxies 1E0657-56. The velocity of the "bullet" subcluster has been measured by X-ray emission from the shock front, and the masses and separation of the main and sub-clusters have been measured by gravitational lensing. The velocity with gravity alone is calculated in a variety of models of the initial conditions, mass distribution and accretion history; it is much higher than expected, by at least 2.4 sigma. The probability of so large a subcluster velocity in cosmological simulations is <~ 10^{-7}. A long range force with strength ~ 0.4 - 1.2 times that of gravity would provide the needed additional acceleration.

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Posts: 131433
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1E 0657-5
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Title: The Cluster-Merger Shock in 1E 0657-56: Faster than the Speeding Bullet?
Authors: Milos Milosavljevic (1), Jun Koda (1), Daisuke Nagai (2), Ehud Nakar (2), Paul R. Shapiro (1) ((1) University of Texas at Austin, (2) Caltech)

Shock waves driven in the intergalactic medium during the merging of galaxy clusters have been observed in X-ray imaging and spectroscopy. Fluid motions inferred from the shock strength and morphology can be compared to the cold dark matter (CDM) distribution inferred from gravitational lensing. A detailed reconstruction of the CDM kinematics, however, must take into account the nontrivial response of the fluid intracluster medium to the collisionless CDM motions. We have carried out two-dimensional simulations of gas dynamics in cluster collisions. We analyse the relative motion of the clusters, the bow shock wave, and the contact discontinuity and relate these to X-ray data. We focus on the "bullet cluster," 1E 0657-56, a near head-on collision of unequal-mass clusters, for which the gas density and temperature jumps across the prominent bow shock imply a high shock velocity 4,700 km/s. The velocity of the fluid shock has been widely interpreted as the relative velocity of the CDM components. This need not be the case, however. 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 profile of the colliding clusters, such a decrease of the inferred halo relative velocity would increase the likelihood of finding 1E 0657-56 in a LambdaCDM universe.

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RE: Dark Matter Discovery
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Title: Can MOND take a bullet? Analytical comparisons of three versions of MOND beyond spherical symmetry
Authors: Garry W. Angus, Benoit Famaey, HongSheng Zhao

A proper test of Modified Newtonian Dynamics (MOND) in systems of non-trivial geometries depends on modelling subtle differences in several versions of its postulated theories. This is especially important for lensing and dynamics of barely virialised galaxy clusters with typical gravity of scale ~ a0 ~ 1 s^-2. The original MOND formula, the classical single field modification of the Poisson equation, and the multi-field general relativistic theory of Bekenstein (TeVeS) all lead to different predictions as we stray from spherical symmetry. In this paper, we study a class of analytical MONDian models for a system with a semi-Hernquist baryonic profile. After presenting the analytical distribution function of the baryons in spherical limits, we develop orbits and gravitational lensing of the models in non-spherical geometries. In particular, we can generate a multi-centred baryonic system with a weak lensing signal resembling that of the merging galaxy cluster 1E 0657-56 with a bullet-like light distribution. We finally present analytical scale-free highly non-spherical models to show the subtle differences between the single field classical MOND theory and the multi-field TeVeS theory.

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Title: Gravitational Lensing in Modified Gravity and the Lensing of Merging Clusters without Dark Matter
Authors: J. W. Moffat

Gravitational lensing in a modified gravity (MOG) is derived and shown to describe lensing without postulating dark matter. The recent data for merging clusters identified with the interacting cluster 1E0657-56 is shown to be consistent with a weak lensing construction based on MOG without exotic dark matter.

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Title: On the Law of Gravity, the Mass of Neutrinos and the Proof of Dark Matter
Authors: Garry W. Angus (1), HuanYuan Shan (2,1), HongSheng Zhao (1,2), Benoit Famaey (3) ((1) University of St. Andrews, (2) NAOC, Beijing, (3) Universite Libre de Bruxelles)

We fit the weak lensing map of the bullet merging galaxy cluster 1E 0657-56 in a class of gravity theories interpolating between GR and MOND (General Relativity and Modified Newtonian Dynamics), so to constrain the nature and amount of dark matter with less dependence on the validity of GR on cluster scales. In agreement with Clowe et al. (2006) we show that a dominant component of non-baryonic matter is needed in the bullet cluster - in MOND as well as in GR. However, the amount of missing matter is consistent with the known inability of a purely baryonic MOND to explain dynamics of other X-ray emitting clusters. The remedy is a "marriage" of MOND with the maximum amount of existing 2eV neutrinos, also invoked in MOND fits of the CMB, which proves acceptable in all clusters. Some issues of consistency with earlier analysis of the bullet cluster are also raised.

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Posts: 131433
Date:
Bullet Cluster
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This collision has swept out the ordinary matter from the Bullet Cluster, more formally known as 1E 0657-56, displacing it with respect to the dark matter (and the galaxies, which act as collisionless particles for these purposes). You can see it directly by superimposing the weak-lensing map and the Chandra X-ray image.

Bullet Cluster
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Posts: 131433
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RE: Dark Matter Discovery
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This composite image shows the galaxy cluster 1E 0657-56, also known as the "bullet cluster." This cluster was formed after the collision of two large clusters of galaxies, the most energetic event known in the universe since the Big Bang.

1E 0657-56

Position(2000): RA 06h 58m 19.85s Dec -55' 56" 29.40
Credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.

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Title: Insights on Physics of Gas and Dark Matter from Cluster Mergers
Authors: Maxim Markevitch, Scott Randall (SAO), Douglas Clowe (University of Arizona), Anthony H. Gonzalez (University of Florida)

I will present constraints on the physical parameters of dark matter and intracluster plasma from the 500 ks Chandra observation of the bullet cluster 1E0657-56 as well as other merging cluster data. In particular, new constraints on the dark matter self-interaction cross-section derived from a combination of X-ray and weak lensing mapping of 1E0657-56 will be presented.

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Rosat HRI imaging suggests that 1E0657-56 is a luminous merger of sub-clusters. A 20 ks Asca Cycle 4 observation resulted in a best-fit temperature of kT=17.5+/-2.5 keV, which makes 1E0657-56 a candidate for the hottest cluster known, and a challenge for cosmological models of galaxy cluster formation.

1E0657-56xray

Position(2000): RA 06h 58m 37.9s | Dec -55 57' 0"
Credit: NASA/SAO/CXC/M.Markevitch et al.

Chandra's image of the extremely hot galaxy cluster 1E 0657-56 in the constellation Carina reveals a bow-shaped shock wave toward the right side of the cluster. This feature, thought to be the result of the merger of a smaller group or sub-cluster of galaxies with 1E 0657-56, gives astronomers a rare opportunity to study how clusters grow.
The shock wave appears to have been formed as 70 million degree Celsius gas in the sub-cluster ploughed through 100 million degree gas in the main cluster at a speed of about 6 million miles per hour. This motion created a wind that stripped the cooler gas from the sub-cluster, similar to leaves from a tree being blown off in a storm.

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