Milky Way Galaxy Wears Two Halos Of Stars

Though it’s devoured and destroyed countless smaller galaxies in its nearly 14-billion-year history, the Milky Way has earned itself two halos of stars, according to a Texas Tech researcher.

Ronald Wilhelm, an assistant professor of physics and co-author of the report, Two Stellar Components in the Halo of the Milky Way, said that though our galaxy is shaped like a flat disk of up to 400 billion stars rotating clockwise, it also wears two crowns of stars that make up a spherical haze and envelops the galaxy’s disk. Though others have suggested that the galaxy wears two halos of stars before, this is the first definitive proof of two different halos, he said.

The report will be published in the Dec. 13 issue of the journal, Nature. It was produced with 11 other researchers working through the Sloan Digital Sky Survey and the Sloan Extension for Galactic Understanding and Exploration.

“These stars in the halos are really, really ancient,” Wilhelm said. “They’re some of the oldest stars in the universe. Probably, this outer halo of stars came about through the cannibalization of smaller galaxies that ran retrograde to our galaxy’s rotation.

“The big deal is that people have suggested for some time this outer halo existed, but they only had small samples of stars in their studies. We’ve created a much larger sample and can see that there is definitely a second halo orbiting retrograde, or backward, to the direction which the disk of the galaxy is rotating. If it was a part of the original formation of our galaxy, that halo should orbit the same direction.”

From a sampling of 20,000 stars, Wilhelm and collaborators determined that the inner halo of stars is more flattened, and orbit slowly but in the same direction as the disk. It remains unclear whether the inner halo arose from mergers of small satellite galaxies or as part of the overall formation of the galactic disk.

The outer halo, however, seems to be composed of a population of stars which were stripped from smaller galaxies that orbited counter to the rotation of the disk. While small galaxies which orbit with our galaxy’s rotation tend to rapidly fall into our galaxy, the retrograde orbiting satellite galaxies dissolve and their stars are spread throughout the outer halo.

By probing the chemical composition of the stars in the second halo, Wilhelm and collaborators were able to determine that these stars came from early protogalaxies or small satellite galaxies that had fewer processed elements such as calcium and iron, when compared to the inner halo.

This change in chemical composition, and orbital characteristics, confirms that two separate populations of stars exist in the halo of the Milky Way. Furthermore, the most likely explanation for the origin of the outer halo stars is that of small, disrupted, satellite galaxies which did not share the chemical history of the Milky Way Galaxy.