In optical light, UGC 1382 appears to be a simple elliptical galaxy (left). When astronomers incorporated ultraviolet and deep optical data (centre) they began to see spiral arms, and when that was combined with a view of low-density hydrogen gas (shown in green at right), scientists discovered that UGC 1382 is gigantic.
At about 718,000 light-years across, UGC 1382 is more than seven times wider than the Milky Way - 10 times larger than was previously thought. But that isn't the strange part.
Whereas most galaxies have the oldest stars closer to the centre, this one is the reverse. "The centre of UGC 1382 is actually younger than the spiral disc surrounding it," says Mark Seibert of the Observatories of the Carnegie Institution for Science, in California. "It's old on the outside and young on the inside. This is like finding a tree whose inner growth rings are younger than the outer rings."
Seibert and Lea Hagen of Pennsylvania State University found the galaxy by accident while they were looking for stars forming in run-of-the-mill elliptical galaxies - of which they thought UGC 1382 was one. But when they started looking more closely at images in ultraviolet light through data from NASA's Galaxy Evolution Explorer (GALEX), they were amazed to see a vast expanse of stars that shouldn't have been there.
"We saw spiral arms extending far outside this galaxy, which no one had noticed before, and which elliptical galaxies should not have," said Hagen, lead author of a study to be published in the Astrophysical Journal. "That put us on an expedition to find out what this galaxy is and how it formed."
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