Bullet cluster

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**Bullet Cluster**
Observation data (Epoch )
Galaxy groups and clusters	List of galaxy groups and clusters
Image:Bullet_cluster.jpg
Constellation(s):	Carina
Right ascension
Declination
Number of galaxies:
Brightest member:
Other designations

The Bullet cluster (1E 0657-56) consists of two colliding clusters of galaxies.<ref>It is not an open cluster or "cluster of stars" which is also sometimes called a "Galactic cluster".</ref> Studies of the Bullet cluster, announced in August 2006, provide the best evidence to date for the existence of dark matter.<ref> Direct constraints on the dark matter self-interaction cross-section from the merging galaxy cluster 1E 0657-56</ref> 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 both clusters' gases to have their velocity changed more than the stars of the two clusters. The third component, the dark matter, was detected indirectly by its 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. It is formally known as 1E0657-56 and has coordinates: RA 06h 58m 37.9s | Dec -55º 57' 0". It is about 4 billion light years from Earth in the constellation of Carina. 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 gas in the main cluster at a speed of about 6 million miles per hour".<ref> Harvard photo and description</ref> <ref>spaceimages.com</ref> <ref> The dynamical status of the cluster of galaxies 1E0657-56</ref> 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<ref> Dark Matter and the Bullet Cluster by Markevitch</ref> and provides "evidence against some of the more popular versions of Modified Newtonian Dynamics (MOND)."<ref> harvard.edu</ref>

Image:060821 darkmatter.jpg

Image:Bullet cluster lensing.jpg

"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 (and interesting) limits on the cross-section of the self-interaction of dark matter."<ref> Recent and Future Observations in the X-ray and Gamma-ray Bands</ref>

"[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."<ref> leidenuniv.nl How Rare is the Bullet Cluster?</ref>

It is a known fact that MOND does not work very well beyond galactic scales. The Bullet Cluster does show that the missing mass is collisionless matter, but it is still not necessarily cold dark matter as proposed in the Lambda-CDM model. This collisionless matter could be massive neutrinos. MOND proponents have long believed that ~2eV neutrinos are needed to explain Cluster data and WMAP data[1]. [2] explains the Bullet Cluster based on ~2eV neutrinos. It could be that cold dark matter explains the MOND phenomenology or a new theory explains it, possibly Tensor-vector-scalar gravity.