Abell 520. Imaged January 2012.
|Observation data (Epoch J2000)|
|Right ascension||04h 54m 03.80s|
|Declination||+02° 53′ 33.00″|
|Number of galaxies||289+|
|Velocity dispersion||1,066 km/s|
|811 Mpc (2,645 Mly) h−1
|ICM temperature||9.8 keV|
|Binding mass||17×1014 h−1
|X-ray luminosity||14.44×1044 erg/s (0.1–2.4 keV)|
|X-ray flux||8.4×10−12 erg s-1 cm−2 (0.1–2.4 keV)|
|The Train Wreck Cluster|
|See also: Galaxy groups, Galaxy clusters, List of galaxy clusters|
The Abell 520 galaxy cluster is an unusual structure resulting from a major merger. It has been popularly nicknamed The Train Wreck Cluster, due to its chaotic structure, and is classified as a Bautz-Morgan type III cluster. It's at a co-moving radial distance of 811 Mpc (2,645 Mly) and subtends 25 arcminutes on the sky.
This particular cluster presents a major problem for prevailing theories about dark matter as well as most alternative theories of modified gravity, because its dark matter content does not appear to behave as expected like in other clusters. As expected, the cluster's galaxies and intergalactic gas content are separated, much like the well-known Bullet Cluster and other merging clusters. It also appears to have as many galaxies and as much intergalactic gas as is expected for a cluster of this size. However, it has a large gravitational lensing core (usually thought to be the location of the dark matter) that appears to be devoid of galaxies or other normal matter. Prevailing theories of Dark Matter believe that dark matter must always stay closely attached to the galaxies. Meanwhile, the prevailing theories also believe that only the intergalactic gas is free to separate out from both the galaxies and the dark matter, during mergers. Here, the dark core appears to have little or no correlation to any of the cluster's other components. It has not yet been determined how the dark matter and galaxies were separated.
A later analysis of the motions of 293 galaxies in the cluster field has suggested that Abell 520 is “a cluster forming at the crossing of three filaments of the large scale structure. The filament aligned with the LOS (line of sight) and projected onto the center of the forming cluster might explain the apparent massive dark core shown by gravitational lensing analysis”. In other words, the dark matter ‘core’ may be associated with a filament aligned with the direction of observation and not be at the centre of the cluster.
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- "‘Cosmic train wreck’ stumps dark-matter physicists". physicsworld.com.
- "Cosmic 'train wreck' defies dark matter theories - space -". New Scientist. Retrieved 16 August 2007.
- "Abell 520: Dark Matter Mystery Deepens in Cosmic "Train Wreck"". Retrieved 16 August 2007.
- Mahdavi, A.; H. Hoekstra, A. Babul, D. Balam, P. Capak (2007). "A Dark Core in Abell 520". Astrophysical Journal 668 (2): 806. arXiv:0706.3048. Bibcode:2007ApJ...668..806M. doi:10.1086/521383.
- Universe Today, Galaxy Cluster Collision Creates a Dark Matter Core
- Dark Matter Core Defies Explanation