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New Evidence of Cannibalism in Andromeda
By Robert Roy Britt
Senior Science Writer
posted: 07:00 am ET
31 March 2004
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EMBARGOED A survey of the nearby Andromeda galaxy and its surroundings has confirmed that the giant spiral is consuming a previously known satellite galaxy. In addition, the study found 14 clusters of stars that appear to be the cosmic fossils of other small galaxies long ago devoured. The work supports the prevailing view that large modern galaxies, including our Milky Way, built their bulk through a form of cannibalism, acquiring the stars of smaller galaxies over billions of years. Around Andromeda is a vast halo of stars is thought to be leftovers from the giant construction project. At first glance, the halo appears to have smooth structure, with stars evenly distributed. Upon closer inspection, the halo is riddled with substructure from past interactions with smaller galaxies. "Given that the disk of Andromeda appears so pristine, we were shocked to discover that its halo shows so much evidence for a history of interactions with other galaxies," said study co-leader Mike Irwin of the University of Cambridge. Irwin's team detected a large stream of stars that's being gravitationally yanked out of a satellite galaxy known as NGC 205, confirming a finding made in 2001 by other researchers. The stream extends nearly 50,000 light years from the core of this small elliptical galaxy. "This is the first clear indication that one of Andromeda's companion galaxies is being ripped apart as we watch," said team member Alan McConnachie, a doctoral student at Cambridge. Andromeda, also called M31, is 2.5 million light-years from the Milky Way. The 14 other newfound star groupings, called globular clusters, each contain thousands of ancient stars. Globular clusters have been detected in the halo of the Milky Way Galaxy and others, and they're thought to be the remains of smaller galaxies that were incorporated into the parents billions of years ago. "Since the most distant of these globular clusters is some 250,000 light-years from the center of M31, our work shows that M31's halo extends far beyond the edge of the bright part of the galaxy disk," said Avon Huxor of the University of Hertfordshire. The researchers say the findings, which were to be presented today at a meeting of the Royal Astronomical Society, will help improve the general understanding of galactic evolution.
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