Using data collected by the Sloan Digital Sky Survey, an international team of astronomers has produced the first-ever chronographic map of the halo of the Milky Way, and discovered that the oldest stars in the galaxy are concentrated in its central region.

Their research, reported last week in the Astrophysics Journal Letters, focused on a sample of 4,700 blue horizontal branch (BHB) stars from throughout the galaxy. The findings can be used to study the properties of older stars and learn more about the chemistry of the early Universe.

Dr. Timothy Beers, one of the study authors and an astrophysicist from the University of Notre Dame, told redOrbit via email that he used spectroscopy data collected during the Sloan Digital Sky Survey to determine the parameters that would allow the team to separate the BHB stars (which have evolved past the red giant stage) from other types of stars.

They used the colors of these BHB stars, which burn helium in the cores, in order to produce an age map of the halo of the Milky Way. Their technique, he explained, “relies on the fact that the colors of BHB stars are related to their masses, which in turn are related to their ages.”

According to Dr. Beers, the chronographic map confirms previous predictions and demonstrates that the oldest stars in the Milky Way are concentrated toward the center of the Galaxy. Perhaps surprisingly, though, he added that the region of the oldest stars extends all the way to the region of the halo closest to the sun, as well as outwards to distances of up to 15 kiloparsecs.

Technique could also be used to identify new star structures

The findings could also be used to identify complex structures of stars, the researchers said. The currently models of galaxy formation show that they continue to be assembled by the process of capturing and shredding smaller dwarf galaxies, which contain stars that are several billion years younger that the first-generation of stars found in the Milky Way, Dr. Beers told redOrbit.

“We have used our map to resolve the ages of a number of known dwarf galaxies and their stellar debris – stripped from them due to their interaction with the Milky Way. This information can be used to tell us the assembly history of our galaxy,” he added. “We can now search for additional debris streams based on their contrast in age, rather than simply density contrast.”

Dr. Beers said that this capability was “almost like having X-ray vision, as we can see through the large numbers of foreground stars and not be distracted by the equally large numbers of background stars, and thereby concentrate on the important structures that inform us about the presence of objects that are contributing to the halo of the galaxy.”

He and his colleagues believe that searching for additional overdensities and debris streams based on age contrast is likely to reveal many new, unrecognized structures. They are currently in the process of preparing a second, more extensive and higher resolution chronographic map of the galactic halo based on several hundred thousand BHB stars.

“This map will be studied in detail by astronomers to identify numerous new dwarf galaxy debris signatures and enable a refined history of the assembly of the Milky Way to be derived,” said Dr. Beers. “Other large samples of BHB stars are being collected by additional surveys, and our technique will be used to study those as well.”

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Feature Image: Thinkstock

Story Image: The darker stars are the oldest. (University of Notre Dame)

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