This Galaxy Has Almost No Dark Matter—And Scientists Are Baffled

If astronomers really have found an "undark" galaxy, it’s a strong clue that dark matter is real.

Published 29 Mar 2018, 10:49 BST
Galaxies litter the cosmos in a picture of the deep universe taken by the Hubble Space ...
Galaxies litter the cosmos in a picture of the deep universe taken by the Hubble Space Telescope. A closer look at a galaxy 65 million light-years away suggests that it is missing a fundamental part of the usual galactic recipe: the mysterious substance known as dark matter.
Photograph by NASA, ESA and the HST Frontier Fields tea

An unusual galaxy far, far away is stumping astronomers not because of what’s there, but because of what’s missing.

What is this thing?

About 65 million light-years away, the galaxy called NGC1052-DF2 is dim and diffuse, coming in at about one two-hundredths the mass of our Milky Way.

Normally, not all of a galaxy’s mass is visible. In addition to a mix of ordinary matter—like stars and planets and manatees—galaxies are expected to contain dark matter, an invisible substance that makes up most of the mass in the universe. Although we can’t directly observe it, we know dark matter is there because we can see how its gravity affects ordinary matter.

Based on the ratio in other galaxies, an isolated galaxy like NGC1052-DF2 should have about a hundred times more dark matter than ordinary matter. But this one appears to have … almost none, scientists reported in Nature.

How did scientists figure that out?

Using a cluster of lenses called the Dragonfly Telephoto Array, a team led by Yale University’s Pieter van Dokkum took a really close look at NGC1052-DF2. By tracking the motion of 10 embedded star clusters, the team could determine how much mass is tucked into the galaxy. And surprisingly, it’s about the same amount of mass they’d expect to see from the galaxy’s stars alone.

“We really thought dark matter was not just an optional component of galaxies,” van Dokkum says, noting that the team has found several other similarly perplexing galaxies to scrutinise.

Why is this observation important?

One strange observation doesn’t necessarily break a theory. But finding a galaxy that’s more or less devoid of dark matter certainly suggests a few tantalising things. First, it really challenges ideas about how galaxies form.

“In modern galaxy formation theory, our understanding is that galaxies form in a dark matter halo,” says Stanford University’s Risa Wechsler. “There’s a pretty tight relationship between the amount of stars that formed and the dark matter there, at least when the galaxy formed.”

In other words, no dark matter, no galaxy.

Van Dokkum and his colleagues outline several scenarios that could explain how this dark matter-free galaxy formed and evolved, but he doesn’t really like any of them. Wechsler is similarly perplexed, and says that if it turns out that galaxies can form in the absence of dark matter, it would be stunning.

“I would need to rethink what a galaxy is,” she says.

Paradoxically, the absence of dark matter in this particular stellar conglomerate also suggests that dark matter does, in fact, exist.

In theories proposing alternatives to dark matter, such as modifications to our understanding of gravity, whatever is mimicking the dark matter signature is not something that can be turned on or off—it should always be there. So, van Dokkum says, “by not detecting the dark matter, we actually prove it’s real.”


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