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Opening Pandora’s Cluster
Posted: 22 June 2011

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The violent and complex past of galaxy cluster Abell 2744, has been pieced together by an international team of scientists using the Hubble Space Telescope and ESO’s ground-based Very Large Telescope, revealing the aftermath of a galactic smash up of at least four separate galaxy clusters.

The resultant mess, known as Pandora’s Cluster, has proven itself to be a galactic box of strange phenomena unleashed by a cosmic crash that took place over a period of roughly 350 million years. “It’s likely that the four merging galaxy clusters were completely separate entities before this merger,” says Julian Merten, one of the lead scientists of the study at the Institute of Theoretical Astrophysics of the University of Heidelberg. “We believe that they are merging together to form a single, massive galaxy cluster.”

Dark matter, X-rays and galaxies in the cluster Abell 2744. The image combines visible light exposures of the galaxy taken by the NASA/ESA Hubble Space Telescope (blue and green) and the European Southern Observatory’s Very Large Telescope (blue, green and red), with X-ray data (shown in pink) from NASA’s Chandra X-ray Observatory and a mathematical reconstruction of the location of dark matter. Image: NASA, ESA, ESO, CXC & D.Coe (STScI)/J. Merten (Heidelberg/Bologna).

Using combined data from the Hubble Space Telescope, the Very Large Telescope, the Japanese Subaru telescope and the Chandra X-ray Observatory, it has been possible to study Abell 2744, which lies so far away from us that light takes approximately 3.5 billion years to reach us, in more detail than ever before. The cluster, which is mostly comprised of dark matter, is unlike any other galactic crash. “The dark matter rarely interacts with normal matter or even itself except through its gravitational pull,” says Dan Coe of the Space Telescope Science Institute, USA. “The gas, however, behaves differently – the gas clouds collide, feel pressure from one another, and get stripped back a bit. This observed separation is one of our strongest pieces of evidence that dark matter exists and is very different from the star-stuff that we are made of.”

The bright galaxies, which are clearly visible in images created by Hubble and the Very Large Telescope, make up less than five percent of the cluster with the majority of Abell 2744 comprising 20 percent hot gas that is visible in X-rays, and 75 percent of invisible dark matter. The mapping of the positions of the three types of matter enabled the team to uncover what was happening in the cluster, with the elusive dark matter being the trickiest to pin down since it does not emit, absorb or reflect light. It is with the help of gravitational lensing that the dark matter could be detected due to the way its powerful gravity magnifies and deflects – or lenses – light from more distant objects behind it.

“Our lensing analysis directly maps the distribution of dark matter and we find four distinct peaks, which tell us how dark matter, gas and galaxies get separated during the merger since they are affected by different physical processes,” says Merten. “Whereas galaxies and dark matter are basically collision-less and don’t feel much from the merger process, the gas feels the hydrodynamical pressure and is usually slowed down.”

Hubble colour image of Abell 2744. Towards the left of the image, one galaxy appears to be “lit up” by its interaction with the cluster’s surrounding gas. While this galaxy has not been studied specifically, scientists can speculate that its gas has been shocked and compressed, triggering a burst of star formation. Newly formed stars burn brightest in the blue and ultraviolet and the galaxies that appear yellow in this image have been stripped of their gas, leaving behind an aging population of yellow and red stars. Image: NASA, ESA, D.Coe (STScI)/J. Merten (Heidelberg/Bologna).

The hot gas in the cluster could be found easily with NASA’s Chandra X-ray Observatory, which also enabled the team to estimate the angles and speeds at which different components of the cluster came together. The results suggest that some of the hot gas and dark matter had been separated and now lie apart from each other and also at a noticeable distance from the visible galaxies.

“The most exciting thing about this collision is the variety of interesting phenomena in one single collision,” says Merten. “We have two bow-shock features and separation between dark matter and gas, like in the famous Bullet Cluster where two clusters collided. We also have a possible cosmic slingshot, where the gas is ejected from the collision and is now preceding the other components.”

It is in the slingshot region of the cluster, where the order of movement, which is seen in the majority of the Abell 2744, is reversed. Near the core of the cluster lies a “bullet” where the gas of one cluster collided with that of another, creating a central shock wave. The team believe that the gas may have been stripped away during the collision, leaving a faint trail of destruction. Moving to the outer regions of the cluster, the distribution is even more peculiar, where one region contains a large amount of dark matter, but no luminous galaxies or hot gas. A clump of ghost-like gas has been ejected, which precedes rather than follows the associated dark matter. The puzzling cluster could be telling astronomers important information about the behaviour of dark matter and how the various ingredients of the Universe interact.

“We are witnessing one of the largest, mega-construction projects in the history of our Universe,” says Coe, another lead author of the study. “Four clusters of galaxies, each with the mass of a hundred trillion Suns, have all been colliding in slow motion for hundreds of millions of years. Ultimately our observations and analysis of this collision will help teach us how these massive structures formed and perhaps about dark matter itself.”

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