Milky Way swallowed a smaller galaxy

By Beau Gamble 12 July 2011
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Australian research is helping to confirm that the Milky Way sucked in a small galaxy at an early stage of its evolution.

NEW RESEARCH IS ADDING evidence to a theory that our galaxy pulled in and swallowed a small neighbour nine billion years ago.

Astronomers in Perth have used a complex computer model to simulate a merger between the infant Milky Way and a smaller galaxy. If future observations of the way that stars move turn out to match what the simulation has predicted, it will provide the strongest evidence yet for the idea of a galactic collision.

To understand how our galaxy was formed, “galactic archaeologists” must understand its present-day shape, says Dr Kenji Bekki, co-author of the research at the International Centre for Radio Astronomy Research (ICRAR), part of the University of Western Australia.

But living deep within the Milky Way makes this challenging, he told Australian Geographic. “We cannot directly see the Milky Way [from Earth], so it’s very difficult to observe these things.”

Galactic archaeology

The model that Kenji created, in collaboration with an international team, depicts what our galaxy would look like if a collision did in fact occur nine billion years ago. Such an event would have created a distinct ‘bar’ shape in the centre of the Milky Way, he says, and if astronomers can observe this bar, it will be clear evidence for a merger. The research is published this week in the Astrophysical Journal.

To envisage the general shape of our galaxy, it’s useful to think of two fried eggs stuck together back-to-back. The yolks are a puffy collection of older stars in the centre, known as the ‘thick disc’. The whites form the ‘thin disc’ – spiral arms of younger stars extending outward. It is on one of these arms that the Earth resides.

According to the simulation, a galactic merger in the Milky Way’s early history would have ‘puffed up’ the thick disc into the shape of a bar. “This is consistent with observation, but there is a lot of work to be done to fully confirm it,” Kenji says.

The next phase of understanding our galaxy’s formation will involve studying the movement of individual stars. “If we can investigate the motion of stars in the thick disc, we can prove that the thick disc is actually a bar,” he says.

Professor Geraint Lewis, an astronomer at the University of Sydney, says complex simulations like Kenji’s are vital for understanding the evolution of the Milky Way.

“We can’t really understand the formation of galaxies by trying to work it out with paper and pen; the interactions are just too complex,” he says. “So these computer simulations are the only real way we have to get a handle on how all the complex processes come together to give us a galaxy.”

When galaxies collide

Galaxies merge when they’re pulled together by gravity, but don’t have enough momentum to keep travelling after the collision. The Milky Way merger must have been a “very high-energy event”, possibly igniting a burst of star formations, Kenji says. “If the galaxies have lots of gas, the merger can trigger a ‘starburst’, so that a lot of new stars are formed.”

Kenji believes that collisions between the Milky Way and other galaxies were, at one stage, actually quite common. “In the very, very early history, lots of smaller galaxies merged with each other to form the Milky Way. But for the last nine billion years, the mergers did not happen too frequently.”

The theory of our galaxy merging with another has been around for about 30 years, but never before have astronomers had a model to test if it is true. Kenji believes real observations may fully confirm his simulation within the next five years.