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Milky Way packed with
alien stars

Posted: 24 February 2010

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Around one-quarter of the globular star clusters hosted by our Milky Way have migrated here from other galaxies, new research finds.

The research, which is accepted for publication in the Monthly Notices of the Royal Astronomical Society, was led by Duncan Forbes of the Swinburne University of Technology in Australia who finds that many stars and globular clusters were born outside of our Milky Way, and migrated into our Galaxy over the last few billion years. "It turns out that many of the stars and globular star clusters we see when we look into the night sky are not natives, but aliens from other galaxies," he says.

A Hubble Space Telescope image of the typical globular cluster Messier 80, an object made up of hundreds of thousands of stars and located in the direction of the constellation of Scorpius. The Milky Way galaxy has an estimated 160 globular clusters of which one quarter are thought to be ‘alien’. Image: NASA / The Hubble Heritage Team / STScI / AURA.

Globular clusters are massive groupings of stars, containing between 10,000 and several million stars each. Astronomers had already suspected that some star clusters might have originated elsewhere, but had been unable to identify which ones. Now, using Hubble Space Telescope Data, Forbes and Canadian collaborator Terry Bridges, were able to determine the age and chemical properties of individual star clusters.

"Using this database we were able to identify key signatures in many of the globular star clusters that gave us tell-tale clues as to their external origin," says Forbes. "The externally formed clusters tend to have younger ages for a given amount of chemical enrichment or metallicity."

Forbes and Bridges determined that these foreign-born globular star clusters actually make up about one quarter of our Milky Way globular star cluster system, implying that tens of millions of stars have joined and added to our Galaxy's stellar population from globular star clusters alone. The work also suggests that the Milky Way may have swallowed up more dwarf galaxies than was previously thought. "We found that many of the foreign clusters originally existed within dwarf galaxies – that is ‘mini’ galaxies of up to 100 million stars that sit within our larger Milky Way," says Forbes.

In the case of the two known dwarf galaxies that have been accreted by the Milky Way, Sagittarius and Canis Major, other astronomers have identified some clusters that share the same dynamical properties, in particular position and velocity. Forbes tells Astronomy Now: "I showed that these clusters also had a particular age-metallicity relation. Other clusters in the Milky Way also showed a similar age-metallicity relation suggesting that they too came from a different dwarf galaxy. I estimated 27-47 clusters were accreted in total from 6-8 dwarf galaxies."

Although the dwarf galaxies are broken-up and their stars assimilated into the Milky Way, the globular star clusters of the dwarf galaxy remain intact and survive the accretion process. "Our research supports the idea that large galaxies like the Milky Way grow by accreting small galaxies and their star cluster systems," says Forbes. "We have also shown that measuring the age-metallicity relation for star clusters in other galaxies can provide important clues to their formation history."