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Galactic cannibalism on our cosmic doorstep

Posted: September 3, 2009

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A new study has found evidence for the Andromeda Galaxy having gobbled up other nearby galaxies, and the Triangulum Galaxy is next on the menu.

An international team of astronomers made the observations of the Andromeda Galaxy – located 2.5 million light years away – as part of an ongoing survey using the Canada-France-Hawaii telescope and its MegaCam/MegaPrime digital camera. The survey is the biggest of its kind, taking in an area with a diameter equivalent to one million light years, the result of which is the broadest and deepest panoramic image of a galaxy ever made.

A projection of the possible orbit of the Triangulum galaxy around Andromeda, taken from the new study. Astronomers believe that Triangulum will eventually be absorbed by its neighbour, contributing to the ongoing formation of Andromeda. Image: University of Cambridge.

The image reveals evidence of the galaxy having digested some of its nearest neighbours in the past. Theories of galaxy formation state that galaxies evolve and grow by absorbing smaller galaxies, but finding evidence for this cannibalistic act is difficult, since structures are often faint and the search requires looking over an area hundreds of times larger than the main disc at the galaxy's centre.

The detail provided in the new study – the first time the outskirts of the galaxy had been explored in such depth – allowed astronomers to find exactly the sort of evidence they needed to support this theory. Furthermore, structures seen on the fringes suggest that Andromeda is still feeding on galaxies today.

"This is a startling visual demonstration of the truly vast scale of galaxies," says Mike Irwin of the University of Cambridge’s Institute of Astronomy. "The survey has produced an unrivalled panorama of galaxy structure which reveals that galaxies are the result of an ongoing process of accretion and interaction with their neighbours."

The scientists say that stars seen in the outermost reaches of the galaxy could not have formed as part of Andromeda itself because the density of gas so far from the galaxy’s core would have been too low to allow formation to take place there. This strengthens the idea that they must be the remains of other smaller galaxies that have been devoured in the relatively recent past, and that Andromeda is still in a state of expansion. 

The new data also suggests that Andromeda has the Triangulum Galaxy lined up for its next course, with interactions already occurring. "Ultimately, these two galaxies may end up merging completely," says team member Scott Chapman. "Ironically, galaxy formation and galaxy destruction seem to go hand in hand."

The results of the survey are published in this week's edition of the journal Nature.