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.”
Hagen and Seibert then looked at data of the galaxy from a range of different telescopes – the Sloan Digital Sky Survey, the Two Micron All-Sky Survey (2MASS), NASA’s Wide-field Infrared Survey Explorer (WISE), the National Radio Astronomy Observatory’s Very Large Array and Carnegie’s du Pont Telescope at Las Campanas Observatory.
Slowly a new model of this mysterious galaxy emerged.
At 250 million light-years away, UGC 1382 turns out to be one of the three largest isolated disc galaxies ever discovered – a rotating disc where stars are slow to form because of the low-density gas that it is made up of.
But the biggest surprise was how the relative ages of the galaxy’s components appear backwards.
The study suggests that this may be because UGC 1382 has been cobbled together with two distinct parts of the galaxy evolving independently before merging. That has resulted in a huge structure, but not a very stable one.
“It is so delicate that a slight nudge from a neighbour would cause it to disintegrate,” says Seibert.
He says it was only able to form and survive because it lies in a quiet part of the universe, making it very rare. More galaxies like this may exist, but research is needed to look for them, the study suggests.
“By understanding this galaxy, we can get clues to how galaxies form on a larger scale, and uncover more galactic neighbourhood surprises,” Hagen says.