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Stripped and cast out, the universe’s loneliest galaxies

By Daniel Clery 23 April 2015 2:00 pm

Compact elliptical galaxies have always been a conundrum. They look like a galaxy stripped bare: as if a normal elliptical galaxy—the sort that is a featureless mass of stars without a spiral structure—has had all its outer stars removed, leaving just the dense core of stars at its center. These rare systems—only a few tens were known until recently—were thought to have had their outer coats of stars ripped away by the gravity of other, larger galaxies as they passed nearby, a theory supported by the fact that they were usually found in the centers of large clusters of galaxies. But in 2013, an isolated compact elliptical galaxy was found, far from any predator galaxies able to rob it of its coat. So how was it created? To find out, astronomers scoured publicly available astronomy databases. They found 195 compact ellipticals; most were in galaxy clusters, but 11 were free fliers, the team reports online today in Science. What’s more, these galaxies had properties just like the others, so they should have had a common origin. The researchers conclude that these outliers were originally in clusters like the others, but after having their outer stars stripped away while orbiting a larger galaxy (orbit shown in this simulation), they had a close encounter with a third galaxy (approaching from bottom) whose gravity flung them out of the cluster like a slingshot. Such a process is known to occur in planetary systems when close encounters can cast a planet into deep space, and within galaxies when a star can get ejected, but these lonely compact galaxies are the result of slingshots on a supergalactic scale.


Source: Science Mag

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This entry was posted on April 27, 2015 by in General and tagged , , , , , , .

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