A galactic crash investigation

Jun 22, 2011
This image combines visible light exposures of galaxy cluster Abell 2744 taken by the NASA/ESA Hubble Space Telescope and the European Southern Observatory’s Very Large Telescope, with X-ray data from NASA’s Chandra X-ray Observatory and a mathematical reconstruction of the location of dark matter. The galaxies in the cluster, while they are the only part that is visible in the optical part of the spectrum, actually only provide around 5 percent of the mass in the cluster. Hot intracluster gas (shown in pink, and responsible for around 20 percent of the mass in the cluster) is visible through its X-ray emissions, observed by NASA’s Chandra satellite. The blue overlay shows a map of the mass in the cluster. This is reconstructed based on detailed analysis of the way that the cluster bends light from galaxies in the distant background. Evidence of this light bending can be seen in arc-like distortions in parts of this image. Since dark matter makes up the lion’s share of mass in the cluster -- around 75 percent -- this blue overlay reveals the location of the otherwise invisible dark matter. Analysis of this data has allowed scientists to observe some strange phenomena in Abell 2744, including a pocket of dark matter with no gas or galaxies, and a clump of galaxies with no associated gas. Astronomers believe that Abell 2744 formed from the simultaneous pile-up of at least four separate clusters. Credit: NASA, ESA, ESO, CXC & D. Coe (STScI)/J. Merten (Heidelberg/Bologna)

(PhysOrg.com) -- A team of scientists has studied the galaxy cluster Abell 2744, nicknamed Pandora’s Cluster. They have pieced together the cluster’s complex and violent history using telescopes in space and on the ground, including ESO’s Very Large Telescope and the Hubble Space Telescope. Abell 2744 seems to be the result of a simultaneous pile-up of at least four separate galaxy clusters and this complex collision has produced strange effects that have never been seen together before.

When huge clusters of galaxies crash together, the resulting mess is a treasure trove of information for astronomers. By investigating one of the most complex and unusual colliding clusters in the sky, an international team of astronomers has pieced together the history of a cosmic crash that took place over a period of 350 million years.

Julian Merten, one of the lead for this new study of cluster Abell 2744, explains: "Like a crash investigator piecing together the cause of an accident, we can use observations of these cosmic pile-ups to reconstruct events that happened over a period of hundreds of millions of years. This can reveal how structures form in the Universe, and how different types of matter interact with each other when they are smashed together."

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This animation shows how the remarkable merger event that created the galaxy cluster Abell 2944, nicknamed Pandora’s Cluster, may have happened. The red regions are clouds of very hot gas, detected by their X-ray emission. The blue clouds are regions rich in dark matter that can only be discovered indirectly by their gravitational influence. This animation shows how the different components behaved over a period of several million years, leading up to their currently observed positions. Credit: ESO/L. Calcada and J. Merten (Heidelberg/Bologna)

"We nicknamed it Pandora's Cluster because so many different and strange phenomena were unleashed by the collision. Some of these phenomena had never been seen before," adds Renato Dupke, another member of the team.

Abell 2744 has now been studied in more detail than ever before by combining data from ESO's Very Large Telescope (VLT), the Japanese Subaru telescope, the NASA/ESA Hubble Space Telescope, and NASA's Chandra X-Ray Observatory.

The galaxies in the cluster are clearly visible in the VLT and Hubble images. Although the galaxies are bright they make up less than 5% of the mass there. The rest is gas (around 20%), which is so hot that it shines only in X-rays, and dark matter (around 75%), which is completely invisible. To understand what was going on in the collision the team needed to map the positions of all three types of matter in Abell 2744.

Dark matter is particularly elusive as it does not emit, absorb or reflect light (hence its name), but only makes itself apparent through its gravitational attraction. To pinpoint the location of this mysterious substance the team exploited a phenomenon known as gravitational lensing. This is the bending of light rays from distant galaxies as they pass through the gravitational fields present in the cluster. The result is a series of telltale distortions in the images of galaxies in the background of the VLT and Hubble observations. By carefully plotting the way that these images are distorted, it is possible to map quite accurately where the hidden mass — and hence the dark matter — actually lies.

By comparison, finding the hot gas in the cluster is simpler as NASA's Chandra X-ray Observatory can observe it directly. These observations are not just crucial to find out where the gas is, but also to show the angles and speeds at which different components of the cluster came together.

When the astronomers looked at the results they found many curious features. "Abell 2744 seems to have formed from four different clusters involved in a series of collisions over a period of some 350 million years. The complicated and uneven distribution of the different types of matter is extremely unusual and fascinating," says Dan Coe, the other lead author of the study.

It seems that the complex collision has separated out some of the hot gas and dark matter so that they now lie apart from each other, and from the visible galaxies. Pandora's Cluster combines several phenomena that have only ever been seen singly in other systems.

Near the core of the cluster is a "bullet", where the gas of one cluster collided with that of another to create a shock wave. The dark matter passed through the collision unaffected.

In another part of the cluster there seem to be galaxies and dark matter, but no hot gas. The gas may have been stripped away during the collision, leaving behind no more than a faint trail.

Even odder features lie in the outer parts of the cluster. One region contains lots of dark matter, but no luminous galaxies or hot gas. A separate ghostly clump of gas has been ejected, which precedes rather than follows the associated dark matter. This puzzling arrangement may be telling astronomers something about how behaves and how the various ingredients of the Universe interact with each other.

Galaxy clusters are the biggest structures in the cosmos, containing literally trillions of stars. The way they form and develop through repeated collisions has profound implications for our understanding of the Universe. Further studies of the Pandora's , the most complex and fascinating merger yet found, are in progress.

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More information: This research is presented in a paper entitled "Creation of cosmic structure in the complex galaxy cluster merger Abell 2744", to appear in Monthly Notices of the Royal Astronomical Society. Research paper: www.eso.org/public/archives/re… /eso1120/eso1120.pdf

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4.4 / 5 (5) Jun 22, 2011
Hmmm... If the colisions have in fact stripped much of the dark matter away from the galaxies in the clusters, then what effect has that had on the rotation of those galaxies? Given that the defining origin of the dark matter theory was "It is required to explain the rotation of stars in galaxies". What theoretical effects can we postulate?
5 / 5 (3) Jun 22, 2011
"Hmmm... If the colisions have in fact stripped much of the dark matter away from the galaxies in the clusters, then what effect has that had on the rotation of those galaxies?"

lengould, I think you may have misunderstood what's reported here. Hot, x-ray emitting gas in the intracluster medium has apparently been stripped from portions of the merging galaxy cluster, similar to what is observed in the Bullet Cluster. Individual cluster members (galaxies) still retain their DM haloes.

The authors mention this mega-merger cluster may be even better suited than the Bullet Cluster in trying to determine several properties of DM.

Fig 9 of the paper has a schematic illustrating the proposed merger scenario for Abell 2744 (very helpful when trying to sort out observations presented here): http://www.eso.or...1120.pdf
5 / 5 (3) Jun 22, 2011
It would be surprising if dark matter didn't exhibit all the gravitational effects that matter exhibits, and the lensing effect probably falsifies MOND-type explanations, IMO.
Lengould100, I suspect that galaxies that have undergone a collision lose the nice, simple rotation associated with spiral galaxies. They become irregular initially.
5 / 5 (1) Jun 22, 2011
I wonder how TeVeS handles this situation.
5 / 5 (2) Jun 22, 2011
Hmmm... If the colisions have in fact stripped much of the dark matter away from the galaxies in the clusters, then what effect has that had on the rotation of those galaxies? ...

Why would the dark matter have to come off the galaxies in the clusters? DM isn't just around galaxies in LCDM. The clusters form around large filaments of DM and intersections in the filaments. Not that some wouldn't come from galaxies.

Galaxy cluster collision doesn't mean galaxies colliding, just as galaxy collision doesn't mean stellar collisions. Sure they will be affected, but they aren't necessarily crashing into each other.

This will be another great testing ground for MOG though. And TeVeS. And MOND. Etc. The biggest problem looking at these clusters is waiting for them to validate theories takes a while. Good thing there are so many.