Stellar Opposites: Sky survey reveals new halo of stars

The Milky Way galaxy possesses a distinct outer halo that orbits in the opposite direction from its inner halo and the rest of the galaxy, researchers say. This second halo contains some of the most primitive stars in the universe, offering new evidence about how the galaxy formed.

Some scientists had previously suspected that a portion of the stars in the Milky Way travel in a different direction from the rest. But data on such stars were too sparse to conclude that an entire second halo existed.

Now, an international team of scientists including Timothy Beers of Michigan State University in East Lansing has discovered stronger evidence for a double halo. Beers and his colleagues analyzed more than 20,000 stars as part of the Sloan Digital Sky Survey, an astronomical effort to create a three-dimensional map of about a million galaxies.

The scientists noticed that stars more than 50,000 light-years away from the center of the Milky Way move in the opposite direction from closer ones, have distinctive chemical compositions, and travel around the galaxy at different speeds.

“It was certainly a surprise to my team how well the two populations were resolved from one another with the new data,” says Beers.

Stars in the Milky Way’s inner disk, where the Earth is located, orbit at 200 kilometers per second (kps). The inner halo moves in the same direction as this disk, but at 25 kps. Stars in the outer halo appear to speed around the galaxy in the opposite direction at about 50 kps.

Beers observes that the existence of small, metal-poor stars in the outer halo suggests a new story of the evolution of the galaxy.

The dense central region of the galaxy and the inner halo that surrounds it probably formed first, says Beers, as heavy, metal-rich stars clumped together to create the Milky Way. Dwarflike galaxies left behind merged together later to create the outer halo, he hypothesizes.

Though this halo formed after the inner halo, its stars are deficient in heavy metals, implying that they are older. These stars likely formed from gas that existed early in the universe, before all elements existed plentifully, says Beers.

The new findings, which appear in the Dec. 13 Nature, do not alter the inference that an invisible form of dark matter occupies the galaxy’s halo region, Beers says. The stars in both halos are so far away from the center of the galaxy that their orbital velocity requires more gravity than visible matter supplies.

Rosie Wyse of Johns Hopkins University in Baltimore says the new data provide compelling statistical evidence in support of a distinct outer halo of the Milky Way. But she says questions remain about the nature and formation of the halo.

For example, the Milky Way’s central bulge contains stars as old as those identified in the outer halo. Wyse says scientists must understand the relationship between this bulge and the outer halo to tell the full history of the galaxy.

“We need future, large, dedicated spectroscopic surveys of stars in our galaxy to answer the many outstanding questions,” she says.

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