Scott Chapman of the California Institute of Technology, Rodrigo Ibata of the Observatoire de Strasbourg, and colleagues said their detailed studies of the motions and metals of nearly 10,000 stars in Andromeda show the galaxy's stellar halo is metal-poor - meaning the stars lying in the outer bounds of the galaxy tend to lack all of the elements heavier than hydrogen.

This is surprising, Chapman said, because one of the key differences thought to exist between Andromeda and the Milky Way was the former's stellar halo was metal-rich and the latter's was metal-poor. If both galaxies are metal-poor, then they must have had similar evolutions.

"Probably, both galaxies got started within a half billion years of the Big Bang, and over the next 3- to 4-billion years, both were building up in the same way by protogalactic fragments containing smaller groups of stars falling into the two dark-matter haloes," he explained.

No one yet knows what dark matter is made of, its existence is well established because of the mass that must exist in galaxies for their stars to orbit the galactic centers the way they do.

Current theories of galactic evolution, in fact, assume that dark-matter wells acted as a sort of "seed" for today's galaxies, with the dark matter pulling in smaller groups of stars as they passed nearby. What's more, galaxies like Andromeda and the Milky Way have each probably gobbled up about 200 smaller galaxies and protogalactic fragments over the last 12-billion years.

Reporting in an upcoming issue of the Astrophysical Journal, Chapman and colleagues reached their conclusion about the metal-poor Andromeda halo by carefully measuring the radial velocity of some of its stars moving either directly toward or directly away from Earth. This measurement can be determined very accurately using the spectrographs of major instruments, such as the 10-meter Keck-II telescope, which was used in the study.

The researchers obtained radial velocities for approximately 10,000 Andromeda stars, including about 1,000 residing in the giant stellar halo that extends outward by more than 500,000 light-years. These stars, because of their lack of metals, are thought to have formed quite early, at a time when the massive dark-matter halo had captured its first protogalactic fragments.

Stars located closer to the center of the galaxy, in contrast, tend to have formed and merged later, and contain heavier elements due to stellar evolution processes.

In addition to being metal-poor, the stars of Andromeda's halo follow random orbits and are not in rotation, while the stars of the visible galactic disk are rotating at speeds upwards of 200 kilometers (about 125 miles) per second.

Ibata said the findings could lead to new insights on the nature of dark matter. "This is the first time we've been able to obtain a panoramic view of the motions of stars in the halo of a galaxy," he said. "These stars allow us to weigh the dark matter, and determine how it decreases with distance."

Other researchers in the study include Geraint Lewis of the University of Sydney in Australia, Annette Ferguson of the University of Edinburgh in Scotland, Mike Irwin of the Institute of Astronomy in Cambridge and Nial Tanvir of the University of Hertfordshire in England, and Alan McConnachie of the University of Victoria in Canada.

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