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Uncovering the Milky Way’s traumatic past

The Milky Way galaxy formed approximately 13.6 billion years ago. Image: NASA/ A. Fujii
The Milky Way galaxy formed approximately 13.6 billion years ago. Image: NASA/ A. Fujii

Astrophysicists in Germany are piecing together the final parts of a map that will help tell us whether the Milky Way suffered a massive collision with another galaxy, billions of years ago.

Our galaxy is pretty average in the scheme of things. Like most other stars in the universe, the stars in the Milky Way are arranged in a wispy, spiral-shaped disc. Our sun is about half way out, on the rim of what is known as the Orion Arm.

Even so, there are some peculiarities with the Milky Way. Simple theories of galaxy formation suggest that its disc of stars ought to be perfectly flat, not puffy as it appears through telescopes.

This has led astrophysicists like Professor Hans-Walter Rix from the Max Planck Institute for Astronomy in Heidelberg to wonder whether some initial turbulence in our corner of the universe prevented a neat formation – or whether a catastrophic collision with another galaxy really stirred things up.

Thanks to the EU-funded MWDISK project, Prof. Rix is attempting to find out. He and his students are creating a map that shows the proportions of chemical elements present in a sizeable portion of our galaxy’s billions of stars.

Those proportions will allow the scientists to determine the age of the stars, so that they can ultimately create something akin to a film strip that reveals the Milky Way’s formation. ‘We see beautiful structures in our galaxy but do we understand how they came about?’ Prof. Rix said. 

Backward equations

In theory, there is a more direct way of finding out about our galaxy’s past than mapping chemical elements and ages. Through observations astronomers can already measure stellar masses and the direction in which stars are moving, so it ought to be possible to follow the equations of motion backwards, like watching a film in rewind.

The trouble is a process known technically as radial-migration. This is where a star essentially swaps its orbit with a nearby star, a bit like a runner passing the baton in a relay race. Observing a star now, it is impossible to tell from its motion whether it is on the same orbit it always had – or whether it stole the ‘baton’ from another star long ago.

Measuring chemical elements, on the other hand, gives an accurate picture of age because heavy elements can only be produced when certain stars reach the end of their lives and explode into supernovae. As time goes on, and more stars turn into supernovae, the proportion of heavy elements in the surroundings rises.

By examining a star’s spectrum – that is, the colours of light it emits – astronomers can work out what proportion of heavy elements it was made up from, and therefore how old it is. 

‘We see beautiful structures, but do we understand how they came about?’

Prof. Hans-Walter Rix, Director, Max Planck Institute for Astronomy, Heidelberg, Germany

Star origins

According to Prof. Rix, these chemical abundances are a bit like a ‘genetic fingerprint’, and they also give astro-detectives like him a way of figuring out where the different stars came from. For example, a star with few heavy elements might well have been born early on, when there was relatively pristine gas and few previous generations of stars.

It is hoped that by putting all this information together, a clearer picture will emerge of how exactly the Milky Way was formed, including the traumas it suffered along the way.

One theory is that the Milky Way’s trauma was fairly mild, just some general turbulence in the early universe. This would mean our galaxy evolved a bit like the city of Paris – growing out from the centre, with suburbs being added concentrically around one another.

On the other hand, some believe the ‘puffiness’ of the Milky Way is the remnant of an ancient collision or merger with another galaxy. This would mean our galaxy evolved more like Berlin, which is the amalgamation of various other historic cities and suburbs.

Prof. Rix hopes he will have sorted through enough information on the chemical abundances to produce a map of Milky Way stars, sorted by age, by the end of next year. ‘That would be qualitatively new, and be the most direct measurement of describing how the Milky Way built up over time,’ he said. ‘Whether we find surprises in there or not, I don’t know.’ 

Dark matter

Prof. Rix’s previous work has already borne fruit on a related question: the amount of dark matter in our cosmic neighbourhood. Dark matter is an invisible substance believed to make up four-fifths of the matter in the universe, yet no-one knows what exactly it is made of.

Dark matter doesn’t emit or absorb light or other electromagnetic radiation, making direct detection extremely difficult. However its existence is inferred from its gravitational effect on other bodies in the universe.

In 2013, Prof. Rix and colleagues managed to put strong limits on the amount of dark matter in our region of the Milky Way. It was less than had been predicted by computer simulations, which might explain – at least in part – why experimental physicists have had so much trouble in directly detecting it.

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