An ultra-faint collection of 1,000 stars orbiting the Milky Way includes the most lightweight galaxies ever discovered, includind Segue 1 and Segue 2. The image above shows a standard prediction for the dark matter distribution within about 1 million light years of the Milky Way galaxy, which is expected to be swarming with thousands of small dark matter clumps called `halos'. The findings, made with the world’s most powerful telescopes offer tantalizing clues about how iron, carbon and other elements key to human life originally formed.
“Segue 1 is so ridiculously metal-poor that we suspect at least a couple of the stars are direct descendants of the first stars ever to blow up in the universe,” says Evan Kirby of the University of California, Irvine. “Segue 1 is the only example that we know of now that was never enriched by these low-mass stars, meaning it formed stars really quickly, in the blink of an eye,” Kirby added. “If it had formed stars long enough those low-mass stars would have to contribute.”
“The big question is, why did it stop?” says U.C. Irvine astrophysicist James Bullock. “A galaxy like this should have been able to make a million more stars, but it didn’t.”
“Maybe Segue 1 was on its way to forming a bunch of stars but reionization turned on and killed all the star formation in the galaxy,” Kirby says. “That could also explain why the star formation lasted such a short time. It’s not obvious to me that reionization by itself could have done this. Maybe, but I definitely think there are other possibilities.”
“Finding a galaxy as tiny as Segue 2 is like discovering an elephant smaller than a mouse,” said Bullock. Astronomers have been searching for years for this type of dwarf galaxy, long predicted to be swarming around the Milky Way. Their inability to find any, he said, “has been a major puzzle, suggesting that perhaps our theoretical understanding of structure formation in the universe was flawed in a serious way.”
Segue 2’s presence as a satellite of our home galaxy could be “a tip-of-the-iceberg observation, with perhaps thousands more very low-mass systems orbiting just beyond our ability to detect them,” he added.
“It’s definitely a galaxy, not a star cluster,” said postdoctoral scholar and lead author Evan Kirby. He explained that the stars are held together by a globule called a dark matter halo. Without this acting as galactic glue, the star body wouldn’t qualify as a galaxy.
Segue 2, discovered in 2009 as part of the massive Sloan Digital Sky Survey, is one of the faintest known galaxies, with light output just 900 times that of the sun. That’s miniscule compared to the Milky Way, which shines 20 billion times brighter. But despite its tiny size, researchers using different tools originally thought Segue 2 was far denser.
The posibility exists that Segue 1 was once a much larger galaxy and lost most of its stars, perhaps through disruptions from the Milky Way. The extremely low metal counts in Segue 1’s stars, however, suggests that it formed roughly the same size it is now, because disruptions would be unlikely to pull only the metal-rich stars from the galaxy, leaving behind the metal-poor.
“W. M. Keck Observatory operates the only telescopes in the world powerful enough to have made this observation,” Kirby said of the huge apparatus housed on the summit of Mauna Kea in Hawaii. He determined the upper weight range of 25 of the major stars in the galaxy and found that it weighs at least 10 times less than previously estimated.
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