A typical star cluster of 1 million stars. The Milky Way has about 160 such clusters and around one quarter of them are 'alien'.

Credit: Hubble Telescope

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BRISBANE: At least a quarter of the star clusters in the Milky Way were born elsewhere and have migrated into our galaxy, researchers say.

The Milky Way may also have swallowed up many more dwarf galaxies than previously thought, according to a report that has been accepted for publication the Monthly Notices of the Royal Astronomical Society.

"[This is] further evidence that our galaxy grows in mass by accreting small galaxies and their star clusters," said Dr Duncan Forbes, an astrophysicist at Swinburne University of Technology in Melbourne, Australia.

Alien clusters are younger

A typical cluster in the Milky Way contains about one millions stars.

Other researchers have estimated the age and chemistry of almost two thirds of the Milky Way's globular clusters, mostly using data on the stars' colours from the Hubble telescope, said Forbes, the study's lead author.

Forbes and his colleagues used these studies to make a database and compare the information on clusters suspected to have been accreted into the galaxy, as well as other clusters with unknown origins.

The globular clusters that were suspected to be alien clusters turned out to be younger than Milky Way-originated clusters, Forbes said.

Eight galaxies swallowed by Milky Way

He also found that at least 27 clusters had the chemical signature suggesting that they formed somewhere other than the Milky Way.

Astronomers had previously confirmed that two dwarf galaxies have been accreted into the Milky Way, but the foreign globular clusters Forbes found suggest that there may have been six more galaxies swallowed up.

Dwarf galaxies are ripped apart by gravity as they are drawn into a larger galaxy, but their globular clusters are more compact and robust, so they survive intact, Forbes said.

Data on the positions and motion of globular clusters collected by the European Space Agency's GAIA spacecraft, due to be launched in 2012, should help test their ideas further, Forbes said.

Sidney van den Bergh, an astrophysicist at Dominion Astrophysical Observatory in Victoria, Canada, said that the work was interesting, but the results could not yet be generalised because of the relatively small number of clusters examined.

"As a result of the well-known perversity of small number statistics we will have to keep in mind the possibility that the apparently clear-cut separation of the data into two branches might still be due to chance," van den Bergh said.

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