Bullet Cluster

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 J2000)
Constellation(s)	Carina
Right ascension	06h 58m 37.9s
Declination	−55° 57′ 0″
Number of galaxies	~40
Redshift	0.296
Distance (co-moving)	1.141 Gpc (3.7 billion light-years).
ICM temperature	17.4 ± 2.5 keV
X-ray luminosity	1.4 ± 0.3 × 1046h50−2 erg/s (bolometric).
X-ray flux	5.6 ± 0.6 × 10−12 erg/cm2/s (0.1–2.4 keV).
Other designations
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.

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.