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UCI scientists size up universe’s most lightweight dwarf galaxy

Segue 2 has a mere 1,000 stars bound by dark matter but could answer a major riddle
The least massive galaxy in the known universe has been measured by UC Irvine scientists, clocking in at just 1,000 or so stars with a bit of dark matter holding them together.

The findings, made with the world’s most powerful telescopes at the W. M. Keck Observatory and published today in The Astrophysical Journal, offer tantalizing clues about how iron, carbon and other elements key to human life originally formed. But the size and weight of Segue 2, as the star body is called, are its most extraordinary aspects.

“Finding a galaxy as tiny as Segue 2 is like discovering an elephant smaller than a mouse,” said UC Irvine cosmologist James Bullock, co-author of the paper. 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 Keck telescopes are the only ones 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.

Fellow authors are Michael Boylan-Kolchin and Manoj Kaplinghat of UC Irvine, Judith Cohen of the California Institute of Technology and Marla Geha of Yale University. Funding was provided by the Southern California Center for Galaxy Evolution (a multicampus research program of the University of California) and by the National Science Foundation.

About the University of California, Irvine: Founded in 1965, UCI is a top-ranked university dedicated to research, scholarship and community service. Led by Chancellor Michael Drake since 2005, UCI is among the most dynamic campuses in the University of California system, with more than 28,000 undergraduate and graduate students, 1,100 faculty and 9,400 staff. Orange County’s second-largest employer, UCI contributes an annual economic impact of $4.3 billion. For more UCI news, visit

News Radio: UCI maintains on campus an ISDN line for conducting interviews with its faculty and experts. Use of this line is available for a fee to radio news programs/stations that wish to interview UCI faculty and experts. Use of the ISDN line is subject to availability and approval by the university.

Janet Wilson | EurekAlert!
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Further reports about: Astrophysical Journal ISDN Milky Way dark matter

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