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Astronomers Use Keck to Study Tiniest Galaxy

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It’s tiny as far as galaxies go, but a galaxy nonetheless, scientists say.

Astronomers from the University of California-Irvine say Segue 2, as the star body is being called, has roughly 1,000 stars.

Compare that to our own Milky Way galaxy, which contains hundreds of billions of stars.

“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 a paper analyzing the find published last week in “The Astrophysical Journal.”

Segue 2, the smallest galaxy identified so far, was discovered in 2009 as part of the Sloan Digital Sky Survey, an eight-year survey of more than a quarter of the sky. The survey created three-dimensional maps containing more than 930,000 galaxies and more than 120,000 quasars.


The Segue 2 galaxyette was analyzed using the twin Keck telescopes atop Mauna Kea.

“Keck Observatory operates the only telescopes in the world powerful enough to have made this observation,” said Evan Kirby, the study’s lead author.

And according to a statement issued by Keck on Friday, it wasn’t an easy task, given its small size and corresponding faint light, roughly equivalent to about 900 times of our nearby sun. By comparison, the Milky Way shines 20 billion times brighter.

But Kirby said his team is convinced that Segue 2 is a galaxy because it is held together by what is known as a dark-matter halo, a sort of “galactic glue” that differentiates galaxies from star clusters.


For years, astronomers have been searching for such dwarf galaxies which were believed to be “swarming” around the Milky Way,” Bullock said.

The previous inability to find any “has been a major puzzle, suggesting that perhaps our theoretical understanding of structure formation in the universe was flawed in a serious way,” he said.

The identification of Segue 2 could be a “a tip-of-the-iceberg observation, with perhaps thousands more very low-mass systems orbiting just beyond our ability to detect them,” Bullock said.

The study’s other authors were 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 multi-campus research program of the University of California, and by the National Science Foundation.

The Keck Observatory operates a pair of cutting-edge 10-meter telescopes under a partnership of the California Institute of Astronomy, the University of California and NASA.

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