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  Cannibalism feeds galactic growth
A new survey has found our nearest galactic neighbour, Andromeda, is beefing itself up by dining out on smaller galaxies.

The findings published today in the journal Nature confirm the theory that galaxies grow by cannibalising smaller systems.

Co-author Professor Geraint Lewis of the University of Sydney's School of Physics, says the survey, which used the Canada-France-Hawaii telescope on Mauna Kea, mapped an area with a diameter of nearly one million light years, centred on Andromeda.

"It's the broadest and deepest image of a galaxy ever made," says Lewis.

When the researchers looked closely at the dwarf galaxy Triangulum, located close to Andromeda, they discovered that it has been weakened by a previous collision with the larger galaxy.

"This little galaxy has had a close encounter with Andromeda and has been shaken up by it," he says.

Lewis says the survey found that millions of Triangulum's stars have been disrupted by the encounter. And the future for Triangulum doesn't get better.

The dwarf galaxy is now orbiting back towards Andromeda, which will see it consumed by its larger neighbour within the next 100 million to billion years, he says.
Growing galaxies

Lewis says the survey confirms the current model of galaxy formation: Galaxies grow in mass through the continual accretion of smaller ones.

The survey also suggests that galaxies may be much larger, with their gravitational influence stretching well beyond stars at the centre of the galaxy.

"We've found coherent structures and star formations over the entire survey area, showing that galaxies are much bigger than we originally thought," Lewis says.

"Andromeda is considered by astronomers to be a typical galaxy, so it's surprising to see how vast it really is."

Lewis says the findings from the survey are important because they inform views on what is happening in the Milky Way.

"One of the big questions in astronomy is where did a galaxy like our Milky Way come from and where is it going?" he says.

He says because of the position of Earth deep within the Milky Way it is difficult for astronomers to understand what occurred in its past.

"Our viewpoint from within the Milky Way introduces selection effects and difficulties in interpretation," he says.

Because Andromeda is "almost a twin" to the Milky Way understanding its history "gives us clues to our [galaxy's] history and where it might be heading".

The future for these giant galaxies isn't good either, with the Milky Way and Andromeda due to collide in three billion years time.
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