Lightest known galaxy is equivalent of mouse-sized elephant, scientist says

Scientists have long thought that the outer Milky Way is dotted with improbably lightweight galaxies - but until now, they haven't been able to find one.

Vincent Thian/AP
This photo taken in June of 2013 shows the Milky Way over Gem Island, on the east coast of Malaysia.

Far, far away, at the Milky Way’s outer skirts, is a galaxy home to just 1,000 stars, all held together with dark matter, our galactic glue.

It’s a little galaxy, but this backwater collection of stars is big news to scientists: it is the lightest dwarf galaxy ever discovered, so incongruously tiny that University of California, Irvine cosmologist James Bullock, co-author of the paper published today in The Astrophysical Journal, likened it to “discovering an elephant smaller than a mouse.”

Scientists have long theorized that the outer Milky Way is packed with lightweight galaxies, but until this newly measured galaxy, called Segue 2, they had been unable to find any examples. Possibly, such small galaxies were too faint, eluding the detection of our earthly equipment. Alternatively, they didn’t exist at all, and “perhaps our theoretical understanding of structure formation in the universe was flawed in a serious way,” Bullock said, in a press release.

When it was first discovered in 2009, scientists thought Segue 2 was significantly more massive than it is now believed to be. But using data from the Keck telescopes, on the summit of Hawaii’s Mauna Kea, scientists have now determined it is at least ten times less than the previously estimated mass.

Segue 2 is also one of the universe’s faintest known galaxies, with a light output just 900 times that of the sun – puny, relative to the Milky Way, which is 20 billion times brighter than that. And this featherweight galaxy differs from a star cluster in that it has structure, its stars are bound together with dark matter.

It’s still possible that Segue 2 is a teeny anomaly in our galactic neighborhood’s outer boroughs. But it also could be "a tip-of-the-iceberg observation, with perhaps thousands more very low-mass systems orbiting just beyond our ability to detect them," Bullock said, in a press release.

That's thousands of mouse-sized elephants out there waiting to be discovered.

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