|Observation data (Epoch J2000)|
|Right ascension||06h 58m 37.9s|
|Declination||−55° 57′ 0″|
|Number of galaxies||~40|
|1.141 Gpc (3.7 billion light-years).|
|ICM temperature||17.4 ± 2.5 keV|
|X-ray luminosity||1.4 ± 0.3 × 1046 h50−2 ergs2/s (bolometric).|
|X-ray flux||5.6 ± 0.6 × 10−12 ergs2/cm2/s (0.1–2.4 keV).|
|1E 0657-56, 1E 0657-558|
|See also: Galaxy groups, Galaxy clusters, List of galaxy clusters|
The Bullet Cluster (1E 0657-558) consists of two colliding clusters of galaxies. Strictly speaking, the name Bullet Cluster refers to the smaller subcluster, moving away from the larger one. It is at a co-moving radial distance of 1.141 Gpc (3.7 billion light-years).
Gravitational lensing studies of the Bullet Cluster are claimed to provide the best evidence to date for the existence of dark matter. However, this interpretation of the gravitational lensing results is disputed (see below).
Observations of other galaxy cluster collisions, such as MACS J0025.4-1222, are similarly claimed to support the existence of dark matter.
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 baryonic, i.e. ordinary, 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 (MOND), 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 (possibly coincident with) the visible galaxies. This provides support for the idea that most of the mass in the cluster pair is in the form of two regions of dark matter, which bypassed the gas regions during the collision. This accords with predictions of dark matter as only weakly interacting, other than via the gravitational force.
The Bullet Cluster is one of the hottest known clusters of galaxies. It provides an observable constraint for cosmological models, which may diverge at temperatures beyond their predicted critical cluster temperature. 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". This energy output is equivalent to that of 10 typical quasars.
Significance to dark matter
The Bullet Cluster provides the best current evidence for the nature of dark matter and provides "evidence against some of the more popular versions of Modified Newtonian Dynamics (MOND)" as applied to large galactic clusters. At a statistical significance of 8σ, it was found that the spatial offset of the center of the total mass from the center of the baryonic mass peaks cannot be explained with an alteration of the gravitational force law alone.
"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."
However, a more recent 2010