THE Milky Way has had a sheltered life. A search for the signs of a violent galactic upbringing has come up empty, and the finding is helping astronomers understand our galaxy's history. It could also aid the search for dark matter.
Galaxies are shape-shifters. Far from being a single set of stars in an eternal spiral, the Milky Way has devoured countless smaller galaxies over its 13 billion year lifetime, and its shape has been in constant flux to accommodate the immigrants. It is riddled with alien stars, threshed from foreign galaxies as they joined ours.
If these galaxies were big hitters, the forces unleashed by a merger would push stars into the heart of our galaxy and create a flattened disc of accreted dark matter that lines up with the spiral disc of stars.
"If you have a disc galaxy and then you have a substantial merger, material is preferentially pulled towards the disc as [the merging galaxy] gets tidally torn to pieces," says Justin Read of the University of Surrey, UK, a co-author of the new study. "Because those shredded satellites will contain stars and dark matter, we can look for evidence that this has happened by picking through the stars in our backyard."
So Greg Ruchti of Lund University in Sweden and his colleagues searched through 4675 Milky Way stars from the Gaia-ESO Spectroscopic Survey looking for remnants of a large merger. They wanted stars with alien chemistry that were a little hotter than Milky Way natives, but rotating at the same speed.
But their galactic archaeological dig found no alien stars in the main disc – although they found plenty on more sedate orbits in the galaxy's suburbs. That suggests that the Milky Way has been steadily gobbling smaller galaxies, but nothing large enough to force new stars and dark matter directly into the disc (http://ift.tt/1cDdcp4).
If the Milky Way lacks a dark matter disc, this has implications for physicists hunting the elusive substance. Dark matter makes up 80 per cent of the universe's matter but only interacts with ordinary matter via gravity, so it is difficult to detect directly.
Earth-based experiments hope a dark matter particle will collide with an ordinary atom in an observable way. Dark matter particles from the galactic disc will give different signals to those from the halo of dark matter the Milky Way was born with –without a disc, the problem is more clear-cut.
Kevin Schawinski of the Swiss Federal Institute of Technology in Zurich says the findings are helpful for comparing the physics of the Milky Way with other galaxies. "The fascinating thing about the Milky Way is that we know it in gorgeous detail because we're in it, but it's also hard to put it in context for the same reason," he says. "We want to know how we got here and how the universe works. Our society and civilisation is the first one that has creation stories that might actually be true. The Milky Way is great, but how did it get that way?"
It would be good to know, since the Milky Way's quiet life is about to end. In 4 billion years, it will collide with its nearest major neighbour, Andromeda, which is roughly the same size and had a much more violent youth. When our stars and dark matter merge, our galaxies' histories will merge too.
Read: "The human universe: Exploring our place in space"
This article appeared in print under the headline "Low-key Milky Way lacks dark matter skeleton"
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