Foraging among the Galaxies: Andromeda’s dining habits are documented

Once a cannibal, always a cannibal.

GALACTIC GLUTTON. Mosaic of the Andromeda galaxy and its surroundings shows 14 newly discovered globular clusters (blue) within the survey region, outlined in red. N. Szymanek and I. King

A new survey is adding to the evidence that Andromeda, the Milky Way’s sister galaxy, has not only grown bigger in the past by feasting on smaller galaxies, but is continuing to do so.

In constructing one of the most detailed images of the large spiral galaxy and a wide region around it, Mike J. Irwin of the University of Cambridge in England and his colleagues found 14 previously unknown globular clusters surrounding Andromeda. These groupings of hundreds of thousands of ancient stars all lie far from Andromeda’s disk. Astronomers have theorized that globular clusters, which are common at the outskirts of large galaxies, are remnants of small galaxies devoured long ago.

Using the 2.5-meter Isaac Newton Telescope in the Canary Islands, the team also found a long stream of stars that appears to have recently been pulled from the small galaxy NGC 205, a well-known satellite of Andromeda. The stream, which lies 50,000 light-years from NGC 205, would be the first found near Andromeda to come from another currently existing galaxy.

The findings “reemphasize the importance and ubiquity of galactic cannibalism in the evolution of galaxies and show that such processes are still ongoing,” says study collaborator Alan W. McConnachie of Cambridge. “The NGC 205 stream probably offers us the best chance of observing a galaxy that is actually in the process of being destroyed,” he adds.

Irwin and his colleagues previously found the first two examples of small-fry galaxies being devoured by the Milky Way (SN: 11/15/03, p. 307: Available to subscribers at Chow Down! Milky Way gobbles its closest known neighbor). Irwin reported the Andromeda findings March 31 at the Royal Astronomical Society’s annual National Astronomy Meeting in Milton Keynes, England.

In a separate study, posted on the Internet (, a team led by Daniel B. Zucker of the Max Planck Institute for Astronomy in Heidelberg, Germany, reports that it has found what looks like another star stream near Andromeda.

To learn more about how galaxies form, astronomers need to determine not only how many star streams are present but also how bright they are and how many came from now-dead satellite galaxies, comments theorist Kathryn V. Johnston of Wesleyan University in Middletown, Conn. The discovery by Irwin’s team “heads us a long way down that path for Andromeda,” she notes.

Johnston adds that a similar survey of the Milky Way’s surroundings, which loom large around us, would require astronomers to survey the entire sky. Given that task, “we might have a better characterization of [star streams and globular clusters] around Andromeda than the Milky Way—at least for the next few years,” she says.

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