Galaxy lightens up (Image: Babak Tafreshi/SPL)

OUR home galaxy has been weighed, and it is surprisingly lean. The latest gauge of the dark matter mass of the Milky Way suggests it weighs only a quarter to a third of the amount previously estimated.

This could explain the dearth of smaller galaxies buzzing around the Milky Way. But it also means we may live in a cosmic anomaly.

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It is thought the first galaxies were born as normal matter coalesced around globs of dark matter, the invisible stuff thought to make up about 80 per cent of the matter in the universe. We can’t see dark matter itself, but we can trace its effects in the motions of stars in modern galaxies.

Stars on the edges of large spirals like the Milky Way are orbiting so fast that they should fly off, so something must be holding on to them. That thing is thought to be a halo of dark matter encircling the visible disc.

Knowing our galaxy’s total mass will tell us a lot about it. “Is our Milky Way typical, or is it actually quite weird?” asks Alis Deason of the University of California, Santa Cruz.

A smattering of stars live in the Milky Way’s dark matter halo, and previous studies have used their motion to figure out the halo’s mass. But we are embedded in a spiral arm, which means dust and gas blocks much of our view of our relatively flat galaxy, so those models had to make assumptions about the parts we can’t see.

To get around the uncertainties, Deason and her colleagues compared two supercomputer simulations that mix different amounts of normal and dark matter to build the Milky Way.

One simulation created a mock Milky Way with a halo as massive as 800 billion suns. The other had a halo weighing 2 trillion suns. The team found that the smaller one was the better fit to actual observations (Astrophysical Journal Letters, doi.org/nd6).

“It is surprising how well it matches the most recent measurements of the properties of the halo of the galaxy,” says Vasily Belokurov of the University of Cambridge, who was not involved in the work.

Galaxies grow by capturing and merging with smaller galaxies. If the Milky Way is as heavy as previously thought, it should be surrounded by thousands of satellites, but we have observed only 26. A lighter Milky Way would command fewer followers, much like what we actually see.

But most galaxies seem to have a ratio of stars to dark matter that falls within a set range, and a lighter halo means the Milky Way is breaking the rules. “That may be pointing to the Milky Way not being the most typical galaxy for its size,” says Deason.

This article appeared in print under the headline “Milky Way sheds mass for latest galactic weigh in”