Discovery of the Musket Ball Cluster

Apr 12, 2012
This composite image shows Chandra (red) and Hubble (yellow and white) data of the galaxy cluster system that has been nicknamed the "Musket Ball" cluster. Astronomers call it this because the newly discovered cluster is older and slower than the Bullet Cluster, a famous system in which "normal" matter has been wrenched apart from dark matter. Chandra detects the normal matter as hot gas, while optical emission reveals the presence of dark matter through the effect of gravitational lensing (blue). DLSCL J0916.2+2951 is further along in its evolution than the Bullet Cluster, giving 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. Credit: X-ray: NASA/CXC/UCDavis/W.Dawson et al; Optical: NASA/STScI/UCDavis/W.Dawson et al.

( -- 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 - 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.

This composite image shows the galaxy cluster 1E 0657-56, also known as the "bullet cluster." This cluster was formed after the collision of two large clusters of galaxies, the most energetic event known in the universe since the Big Bang. Credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.

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 . 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. Findings on the Musket Ball Cluster were presented at the American Astronomical Society meeting in Austin, TX in January by Will Dawson from the University of California, Davis.

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More information: A paper describing these results led by Dawson was published in the March 10, 2012 issue of The Astrophysical Journal Letters.

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3 / 5 (4) Apr 12, 2012
Ok, this is very convincing that Dark Matter is a "something," i.e. a field, particle, or some other invisible or ultra-small abstract or extended object, but it does not yet prove it's nature exactly.

I would have to admit that this finding cannot be explained by a simple error in gravitational formula.

This is what you would expect if the DM halos were moving at the same velocity as the original galaxy clusters. Because the ordinary matter interacts and collides with itself, "friction" and other forms help gravity hold it together in bigger and bigger clumps.

Meanwhile, the DM passes through itself and most of the ordinary matter, and therefore is ejected temporarily, or perhaps will be found to be in a very, very large pogo orbit. Perhaps it will eventually fall back into the cluster eons in the future and be captured by black holes and such...

If they had a complete pass through, the black holes in each galaxy cluster should have grown enormously during the event.
3 / 5 (4) Apr 12, 2012
There may be many "intermediate mass" black holes, in the range of 100's to several 1000's of solar masses in these clusters after this event.

A complete "blow through" like that should have fed enormous amounts of gas and dust from ordinary matter, and even DM, to all of the black holes, both the SMBH size and the "stellar" size and even the stars in the various galaxies.

After another pass or two, you might expect to find a "big crunch" effect, with whole galaxies falling into one another and forming a super-galactic mass black hole.

Perhaps that what they should look for next: A super-galactic black hole that is still young enough to be detected from it's jets or other radiation sources.

Doing a survey to find something like that would require a full survey of the entire sky at this range and resolution or perhaps a bit "older".
1 / 5 (3) Apr 12, 2012
Non-baryonic dark matter might just be neutrinos. We already have good estimates of the mass difference between neutrino flavours, but we don't know the neutrino mass yet. If the mean neutrino mass is on the order 4 eV/c^2 then that would be sufficient to provide all the non-baryonic DM.
2 / 5 (4) Apr 12, 2012
In fact, the writer of the article, if not also the "researchers" contributing, don't seem to agree on what "normal matter" and "dark matter" mean. In the caption for the Musket Ball Cluster, it states that Chandra data distinguishes the rpesence of "normal matter" by the presence of hot gas, while "dark matter" is indicated by gravitational lensing. For the Bullet Cluster, however, hot gas again is depicted by data from Chandra, and "matter" is revealed by "the effects of gravitational lensing", predicted by Einstein. Leaving aside the fact that there are no signs of lensing in either of the images shown, they don't seem to know whether to talk about normal matter or "dark matter". Einstein said even normal matter causes lensing! Nor do they realize lensing comes from concentrations of matter, not an even distribtuion, which is how "dark matter" is usually represented.
1 / 5 (3) Apr 13, 2012
It's not dark matter. It's a reduction in density of anti-gravity matter.