http://www.space.com/scienceastronomy/milky-way-ancient-stars-collisions-100701.html
Some Ancient Stars In Milky Way Were Born Elsewhere By Zoe Macintosh SPACE.com Staff Writer posted: 01 July 2010 Our Milky Way galaxy snatched up many of its most ancient stars from smaller galaxies that shredded each other in violent collisions, a new study suggests. Using new supercomputer simulations, researchers found that some ancient Milky Way stars did not form natively with the rest of the galaxy about 10 billion years ago. Instead, they are actually the leftovers from other galaxies that collided about 5 billion years ago. These stars make up some of the residents in the Milky Way's stellar halo, which extends above and below the spiral galaxy's main disk, researchers said. "Effectively we became galactic archaeologists, hunting out the likely sites where ancient stars could be scattered around the galaxy," said researcher and post graduate student Andrew Cooper of the Institute of Computational Cosmology at Durham University in the United Kingdom. "Like ancient rock strata that reveal the history of Earth, the stellar halo preserves a record of a dramatic primeval period in the life of the Milky Way which ended long before the sun was born," he added. The new simulations began about 13 billion years ago, just after the universe began, and then used universal laws of physics to chart how the gravitational attraction of the galaxies' dark matter halos accumulated stars over time. Dark matter halos are regions of invisible matter around a galaxy which astronomers only infer exist because of their gravitational effects on visible matter. "The simulations are a blueprint for galaxy formation," said researcher Carlos Frenk, director of the Institute of Computational Cosmology at Durham. Frenk said the simulations reveal clues into the "early, violent history" of the Milky Way galaxy. Researchers credited the unusually high resolution of the new simulation for its results. Capable of zooming in on the fine details of galaxy unraveling, the simulations showed streams of stars being pulled from clusters due to the gravity of hidden dark matter. Most surprising was the similarity between the simulated results and real observations, said Cooper. "This shows that the cold dark matter model gives a convincing match to the real universe not just on very large scales, but also for individual Milky Way-like galaxies," he told SPACE.com. The cold dark matter model is the current "standard model" of cosmology, which depicts galaxy formation in the universe as a process primarily arising from the clumping of dark matter. The research is detailed in the journal Monthly Notices of the Royal Astronomical Society.
