Bullet Cluster

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Bullet Cluster

X-ray photo by Chandra X-ray Observatory. Exposure time was 140 hours. The scale is shown in megaparsecs. Redshift (z) = 0.3, meaning its light has wavelengths stretched by a factor of 1.3.
Observation data (Epoch )
Constellation(s) Carina
Right ascension 06h 58m 37.9s
Declination -55º 57' 0"
See also: Galaxy groups and clusters, List of galaxy clusters
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The Bullet cluster (1E 0657-56) consists of two colliding clusters of galaxies.[1] Studies of the Bullet cluster, announced in August 2006, provide the best evidence to date for the existence of dark matter.[2] The major components of the cluster pair, stars, gas and the putative dark matter, behave differently during collision, allowing them to be studied separately. The stars of the galaxies, observable in visible light, were not greatly affected by the collision, and most passed right through, gravitationally slowed but not otherwise altered. The hot gas of the two colliding components, seen in X-rays, represents most of the mass of the ordinary (baryonic) matter in the cluster pair. The gases interact electromagnetically, causing the gases of both clusters to slow much more than the stars. The third component, the dark matter, was detected indirectly by the gravitational lensing of background objects. In theories without dark matter, such as Modified Newtonian Dynamics, the lensing would be expected to follow the baryonic matter; i.e. the X-ray gas. However, the lensing is strongest in two separated regions near the visible galaxies. This provides support for the idea that most of the mass in the cluster pair is in the form of collisionless dark matter.

The Bullet cluster is one of the hottest known clusters of galaxies. Observed from Earth, the subcluster passed through the cluster center 150 million years ago creating a "bow-shaped shock wave located near the right side of the cluster" formed as "70 million degree Celsius gas in the sub-cluster plowed through 100 million degree Celsius gas in the main cluster at a speed of about 6 million miles per hour".[3] [4] [5] Strictly speaking, the name Bullet cluster refers to the smaller subcluster, moving away from the larger one.

It provides the best current evidence for the nature of dark matter[6] and provides "evidence against some of the more popular versions of Modified Newtonian Dynamics (MOND)."[7]

Photo from NASA of the Bullet Cluster showing the inferred dark matter distribution as blue and the measured hot gas distributions in red.
Photo from NASA of the Bullet Cluster showing the inferred dark matter distribution as blue and the measured hot gas distributions in red.
Mass density contours superimposed over photograph taken with Hubble Space Telescope.
Mass density contours superimposed over photograph taken with Hubble Space Telescope.

"Particularly compelling results were inferred from the Chandra observations of the 'bullet cluster' (1E0657-56; Fig. 2) by Markevitch et al. (2004) and Clowe et al. (2004). Those authors report that the cluster is undergoing a high-velocity (around 4500 km/s) merger, evident from the spatial distribution of the hot, X-ray emitting gas, but this gas lags behind the subcluster galaxies. Furthermore, the dark matter clump, revealed by the weak-lensing map, is coincident with the collisionless galaxies, but lies ahead of the collisional gas. This—and other similar observations—allow good limits on the cross-section of the self-interaction of dark matter."[8]

"[T]he velocity of the bullet subcluster is not exceptionally high for a cluster substructure, and can be accommodated within the currently favoured Lambda-CDM model cosmogony."[9]

[edit] Sources and notes

  1. ^ It is not an open cluster or "cluster of stars" which is also sometimes called a "Galactic cluster".
  2. ^ Direct constraints on the dark matter self-interaction cross-section from the merging galaxy cluster 1E 0657-56
  3. ^ Harvard photo and description
  4. ^ spaceimages.com
  5. ^ The dynamical status of the cluster of galaxies 1E0657-56
  6. ^ Dark Matter and the Bullet Cluster by Markevitch
  7. ^ harvard.edu
  8. ^ Recent and Future Observations in the X-ray and Gamma-ray Bands
  9. ^ leidenuniv.nl How Rare is the Bullet Cluster?

[edit] Further reading