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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

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of at least four separate galaxy clusters. 

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This is the ESOcast!

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Cutting-edge science and life behind the scenes of ESO,

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the European Southern Observatory.

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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

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are clearly visible in images 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 mass in the cluster, about 75%, is dark matter.

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The Dark matter cannot be seen directly,

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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 that 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,

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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 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 

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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

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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,

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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

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how dark matter behaves and how the various ingredients of the Universe

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 interact with each other.

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Galaxy clusters are the largest objects in the Universe

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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

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the nature of this mysterious and elusive substance.

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ESOcast is produced by ESO, the European Southern Observatory.

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ESO, the European Southern Observatory, is the pre-eminent intergovernmental science and technology organisation in astronomy,

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designing, constructing and operating the world’s most advanced ground-based telescopes.

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Transcription by ESO ; translation by —

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Now that you've caught up with ESO,

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head 'out of this world' with Hubble.

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The Hubblecast highlights the latest discoveries of the world´s most recognized and prized space observatory,

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the NASA/ESA Hubble Space Telescope