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DLSCL J0916.2+2951: Discovery of the Musket Ball Cluster
DLSCL J0916.2+2951
DLSCL J0916.2+2951
DLSCL J0916.2+2951

  • A system of colliding galaxy clusters, nicknamed the "Musket Ball" cluster, has been discovered.

  • Astronomers call it this because it is an older and slower cousin to the famous Bullet Cluster, where "normal" and dark matter have been torn apart.

  • This image shows the Musket Ball Cluster at about 700 million years post-collision, showing it is much older than the Bullet Cluster.

  • Finding this cluster gives scientists insight into a different phase of how galaxy clusters grow and change after major collisions.

Using a combination of powerful observatories in space and on the ground, astronomers have observed a violent collision between two galaxy clusters in which so-called normal matter has been wrenched apart from dark matter through a violent collision between two galaxy clusters.

The newly discovered galaxy cluster is called DLSCL J0916.2+2951. It is similar to the Bullet Cluster, the first system in which the separation of dark and normal matter was observed, but with some important differences. The newly discovered system has been nicknamed the "Musket Ball Cluster" because the cluster collision is older and slower than the Bullet Cluster.

Finding another system that is further along in its evolution than the Bullet Cluster gives scientists valuable insight into a different phase of how galaxy clusters - the largest known objects held together by gravity - grow and change after major collisions. Researchers used observations from NASA's Chandra X-ray Observatory and Hubble Space Telescope as well as the Keck, Subaru and Kitt Peak Mayall telescopes to show that hot, X-ray bright gas in the Musket Ball Cluster has been clearly separated from dark matter and galaxies.

In this composite image, the hot gas observed with Chandra is colored red, and the galaxies in the optical image from Hubble appear as mostly white and yellow. The location of the majority of the matter in the cluster (dominated by dark matter) is colored blue. When the red and the blue regions overlap, the result is purple as seen in the image. The matter distribution is determined by using data from Subaru, Hubble and the Mayall telescope that reveal the effects of gravitational lensing, an effect predicted by Einstein where large masses can distort the light from distant objects.

In addition to the Bullet Cluster, five other similar examples of merging clusters with separation between normal and dark matter and varying levels of complexity, have previously been found. In these six systems, the collision is estimated to have occurred between 170 million and 250 million years earlier.

In the Musket Ball Cluster, the system is observed about 700 million years after the collision. Taking into account the uncertainties in the age estimate, the merger that has formed the Musket Ball Cluster is two to five times further along than in previously observed systems. Also, the relative speed of the two clusters that collided to form the Musket Ball cluster was lower than most of the other Bullet Cluster-like objects.

The special environment of galaxy clusters, including the effects of frequent collisions with other clusters or groups of galaxies and the presence of large amounts of hot, intergalactic gas, is likely to play an important role in the evolution of their member galaxies. However, it is still unclear whether cluster mergers trigger star formation, suppress it, or have little immediate effect. The Musket Ball Cluster holds promise for deciding between these alternatives.

The Musket Ball Cluster also allows an independent study of whether dark matter can interact with itself. This information is important for narrowing down the type of particle that may be responsible for dark matter. No evidence is reported for self-interaction in the Musket Ball Cluster, consistent with the results for the Bullet Cluster and the other similar clusters.

The Musket Ball Cluster is located about 5.2 billion light years away from Earth. A paper describing these results was led by Will Dawson from University of California, Davis and was published in the March 10, 2012 issue of The Astrophysical Journal Letters. The other co-authors were David Wittman, M. James Jee and Perry Gee from UC Davis, Jack Hughes from Rutgers University in NJ, J. Anthony Tyson, Samuel Schmidt, Paul Thorman and Marusa Bradac from UC Davis, Satoshi Miyazaki from the Graduate University for Advanced Studies (GUAS) in Tokyo, Japan, Brian Lemaux from UC Davis, Yousuke Utsumi from GUAS and Vera Margoniner from California State University, Sacramento.

NASA's Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory controls Chandra's science and flight operations from Cambridge, Mass.

Fast Facts for DLSCL J0916.2+2951:
Credit  X-ray: NASA/CXC/UCDavis/W.Dawson et al; Optical: NASA/STScI/UCDavis/W.Dawson et al.
Release Date  April 12, 2012
Scale  6.4 arcmin across (about 8 million light years)
Category  Groups & Clusters of Galaxies
Coordinates (J2000)  RA 09h 16m 14.64s | Dec +29° 54´ 24.00"
Constellation  Cancer
Observation Date  2 Jan 2011
Observation Time  11 hours 6 min
Obs. ID  12913
Instrument  ACIS
References Dawson, W. et al, 2012, ApJ 747, 42; arXiv:1110.4391
Color Code  Optical (Red, Green, Blue); X-ray (Red-Purple); Mass Map (Blue)
Distance Estimate  About 5.23 billion light years
distance arrow
Visitor Comments (10)

Dear Raman,
Thanks for your good question. On relatively small scales, not much bigger than a galaxy, the expansion of the Universe is not large enough to overcome the attractive force of gravity. So, if galaxies are reasonably close to each other, gravity will pull them together and if they're not, the expansion wins.
-P. Edmonds, CXC

Posted by P. Edmonds on Friday, 11.9.12 @ 11:23am

Dear Lary Kim,
Thanks for your interesting question. Dark energy is not thought to be a byproduct of matter and antimatter collisions, since these events result in radiation. On the other hand dark energy, as the name suggests, is dark. For more details about the leading models for dark energy, please see this web-page:
-P. Edmonds, CXC

Posted by P. Edmonds on Friday, 11.9.12 @ 11:19am

What is 1/2 life of gamma radiation from a matter/antimatter incident?
If enough time has passed to render matter on Earth inert but leave behind gold, carbon & iron without harmful radiation... could not the same occur with dark energy if it is a byproduct of m/am events?

Posted by Lary Kim on Thursday, 10.25.12 @ 11:39am

If the big bang and inflation is correct why do the galaxies collide when the space between them go on increasing?

Posted by Dr. Raman Mannazhi M. D. on Saturday, 10.13.12 @ 21:09pm

I think if theory of big bang is correct, collusion should not happen. It can happen only if there was various big bangs on different time scales.

Posted by Sanjay Jain on Thursday, 10.4.12 @ 05:11am

How does dark matter help to explain gravitational parallax?
"A typical microlensing event like this one has a very simple shape, and only one physical parameter can be extracted: the time scale, which is related to the lens mass, distance, and velocity. There are several effects, however, that contribute to the shape of more atypical lensing events:
Parallax. For events lasting for months, the motion of the Earth around the Sun can cause the alignment to change slightly, affecting the light curve."

Posted by Allen on Tuesday, 09.18.12 @ 20:24pm

I find this extremely useful and impressive. I would like to know more about dark matter and how it interacts with itself.

Posted by Hafez on Wednesday, 05.9.12 @ 03:40am

Hi Tonia Hardy,
Type in any search engine Dark Matter.
You will get all kinds of web sites on the subject. Go into the catalog of your local library and type in Dark Matter in the key word section of the online catalog and you should find plenty of books on the subject.
Marvin L. S.

Posted by Marvin L. S. on Sunday, 04.22.12 @ 23:34pm

I find this extremely useful and impressive. I would like to know more about dark matter and how it interacts with itself.

Posted by Tonia Hardy on Thursday, 04.12.12 @ 13:55pm

If the cluster is appears to be moving slowly, maybe it is due to the relatively shallow angle between the various galaxies.

Posted by Brian Pearson on Thursday, 04.12.12 @ 13:43pm

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