Smallest galaxy sheds light on evolution of dark matter in Universe

Astronomers have discovered a strangely tiny galaxy in the Milky Way’s neighborhood -- one with less than 1,000 stars held together by the smallest dark matter halo ever observed.

The galaxy known as Segue 2, described in the Astrophysical Journal, might hold the key to a long-standing mystery about the evolution of the universe.

“These little clumps are almost certainly the first things to form in the universe,” said study coauthor James Bullock, an astronomer at UC Irvine.

Segue 2, discovered by an extension of the Sloan Digital Sky Survey in 2009, is putting out about as much light as 900 suns, Bullock said. The Milky Way galaxy, by contrast, contains about 100 billion stars. In fact, Segue 2 is so small that it’s dwarfed by many star clusters, which are simply collections of stars inside a galaxy that can contain 100,000 solar masses.

Astronomers have come to realize that size isn’t the key difference between a star cluster and a tiny galaxy. Unlike a star cluster, all galaxies great and small are filled with and surrounded by a halo of dark matter -- the invisible, mysterious stuff that fills the universe and acts as a sort of glue within and between galaxies. Thin tendrils of dark matter connect nodes of galaxy clusters, creating a cosmic web that has given the universe its structure.

Theorists are still trying to understand how this structure evolved, figuring that there must have been a kind of early intermediate stage when tiny dark matter clumps formed, grew, pulled in stars and other visible matter and finally became the giant masses we see today.

Some of those tiny pockets of dark matter should have survived as “fossil remnants” that never grew past that initial stage, Bullock said.

But astronomers had been unable to find them, because they’d need to find one marked by a tiny, and thus exceedingly dim, galaxy. They had started to wonder if this crucial link in the theories might be wrong.

“It’s almost like panning for gold … sifting through the stars in the sky to find these gems, these little galaxies,” Bullock said.

But then Segue 2 showed up. And while it doesn’t have the lowest number of stars, it boasts the smallest dark matter halo ever seen. It holds so little dark matter, in fact, that the researchers couldn’t get an entirely accurate gauge on its mass, setting the upper limit at less than 100,000 solar masses -- chump change for most galaxies.

Even though Segue 2 is exceedingly small, it contains a complex ecosystem of stars in which some of the newer stars were enriched by heavy elements from older stars that exploded into supernovae.

It’s unclear whether Segue 2 is simply a strange, miniaturized fossil that never grew into a larger galaxy in a larger dark matter cluster, or whether it’s the remnants of a larger galaxy that’s been shredded by the gravitational pull from the massive Milky Way.

The researchers would need to find and study more such ultrafaint galaxies to find out -- and to show that this mini-galaxy isn’t an anomaly. If it’s not a common occurrence throughout the universe, then perhaps such morsels of dark matter were not the missing link in the evolution of the universe’s dark matter structure, Bullock said, and scientists might have to start revising their theories.

“It would also probably mean that the dark matter particle, the fundamental microphysical nature of what the dark matter is, is different than is sort of the standard paradigm now,” Bullock said. “That would be really exciting, actually, if that is the case.”

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