Ghostly Galaxies Are Light on Stars But Heavy on Dark Matter

By Ken Croswell

Published on New Scientist (March 22, 2016)

There’s more than meets the eye. Astronomers have weighed a so-called ultra-diffuse galaxy for the first time, and found that it is over 99.96 per cent dark matter.

An ultra-diffuse galaxy can be as large as the Milky Way but as dim as a dwarf. The galaxy’s few stars are spread out, so it looks ghostly, making it hard to study.

Although observers spotted the first few examples three decades ago, they didn’t have a name until 2014, when a team led by Pieter van Dokkum at Yale University discovered 47 of them in the Coma galaxy cluster. Other astronomers studied this cluster with the giant Subaru Telescope in Hawaii and found hundreds more.

Van Dokkum’s team argued that the galaxies had to consist of at least 98 per cent dark matter for gravity to hold them together. Otherwise, the many other galaxies in the Coma cluster would tear them apart.

Dark matter is thought to make up about 80 per cent of the mass in the universe overall, so that would be an impressively dense concentration of the stuff in a small space. But until now no one had directly measured an ultra-diffuse galaxy’s mass.

Now Michael Beasley at the Institute of Astrophysics of the Canary Islands, Spain, and his colleagues have weighed an ultra-diffuse galaxy in the Virgo cluster named VCC 1287. The galaxy’s main body is too dim to study easily, so instead they observed seven of its globular clusters – bright, tight-packed gatherings of stars that move around the galaxy.

Dim but heavy

Using the 10.4-metre Great Canary Telescope in La Palma, Beasley’s team measured the clusters’ speeds. They found that the clusters orbit the galaxy so fast that it must have about 80 billion times more mass than the Sun. That’s only 8 per cent of the Milky Way’s total mass, but an impressive figure for a galaxy that has so few stars that it emits less than a thousandth as much light.

It also means the galaxy has 3000 times more dark matter than stellar mass. The Milky Way’s ratio is just 15 to 1.

“It’s a very clever method,” says van Dokkum. “It’s a great achievement.”

Beasley speculates that the galaxy he studied was born with both dark matter and gas but lost the latter as the galaxy fell into the Virgo cluster. Without gas, the galaxy couldn’t create new stars, so it ended up with lots of dark matter but little light.

Ken Croswell earned his Ph.D. in astronomy from Harvard University and is the author of The Alchemy of the Heavens and The Lives of Stars.

"An engaging account of the continuing discovery of our Galaxy...wonderful." --Owen Gingerich, The New York Times Book Review. See all reviews of The Alchemy of the Heavens here.

"A stellar picture of what we know or guess about those distant lights."--Kirkus. See all reviews of The Lives of Stars here.

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