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A team of scientists has studied the unusual galaxy cluster Abell 2744, 

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nicknamed Pandora’s Cluster.

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Using telescopes in space and on the ground, 

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including the NASA/ESA Hubble Space Telescope 

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and ESO’s Very Large Telescope, 

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they have discovered that it was formed by the simultaneous pile-up of 

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at least four separate galaxy clusters. 

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Episode 47: Pandora’s Cluster

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When huge clusters of galaxies crash together, 

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the resulting mess is a treasure trove of information. 

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Observing these cosmic pile-ups lets astronomers reconstruct events 

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that have happened over hundreds of millions of years. 

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It also lets them study how different types of matter behave during these collisions.

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The galaxies in Pandora’s Cluster are clearly visible in images 

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from Hubble and ESO’s Very Large Telescope,

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but they only make up about 5% of the cluster’s mass.

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About 20% is hot gas, which is shown here in pink. 

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This gas is visible thanks to its X-ray emission 

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which can be detected with NASA’s Chandra satellite. 

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The lion’s share of the mass in the cluster, about 75%, is dark matter. 

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The dark matter cannot be seen directly, and is somewhat of a mystery in modern day astronomy. 

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But there is a trick astronomers can use to find out its location:

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by looking at the way gravity distorts light from more distant galaxies, 

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they can work out where dark matter is hiding.

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This lets the astronomers make a detailed map of where the dark matter is, shown here in blue. 

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Comparing the location of the galaxies, the hot gas and the dark matter, 

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shows that this is not a simple crash between two clusters.

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By reconstructing the history of Pandora’s Cluster, 

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astronomers think that it must have formed from four different clusters

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involved in a series of collisions over a period of some 350 million years. 

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It seems that the complex collision has separated out some of the hot gas and dark matter 

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so that they now lie apart from the visible galaxies.

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Near the core of Abell 2744,

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the gas of one cluster has collided with that of another to create a shock wave. 

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The dark matter passed through the collision unaffected.

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In another part of the cluster there seem to be galaxies and dark matter, but no hot gas. 

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The gas may have been stripped away during the collision, 

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leaving behind no more than a faint trail.

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Even stranger features lie in the outer parts of the cluster. 

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One region contains lots of dark matter, but no luminous galaxies or hot gas. 

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A separate ghostly clump of gas has been ejected,

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which precedes rather than follows the associated dark matter. 

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This puzzling arrangement may be telling astronomers something about how dark matter behaves 

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and how the various ingredients of the Universe interact with each other.

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Galaxy clusters are the largest objects in the Universe to be held together by their own gravity 

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and understanding how they form and evolve 

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is a vital aspect of unravelling the history of the cosmos. 

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Learning more about dark matter not only furthers our understanding of clusters, 

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but it also takes us a little closer to fathoming the nature of this mysterious and elusive substance.

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Transcribed by ESA/Hubble