Astronomers have found a peculiar object: a fluffy galaxy that has little to no dark matter.
You can’t have a cup of coffee without the cup. Astronomers have long thought the same goes for galaxies and dark matter: the dark matter forms the cup, and the galaxy coalesces from the gas that pours into that cup, pulled there by gravity.
The fluffy galaxy NGC 1052-DF2 makes no sense in this picture: it’s the coffee without the cup. Using an impressive collection of ground- and space-based observations, Pieter van Dokkum (Yale) and colleagues have found that this diffuse galaxy has at most 1⁄400 as much dark matter as expected, based on other systems of similar mass. In fact, the best explanation may be that there’s no dark matter at all.
The peculiar object is one of a collection of galaxies dominated by the beefy elliptical NGC 1052, which lies some 65 million light-years away on the border of the constellations Cetus and Eridanus. Astronomers already knew DF2 existed, as it showed up as a collection of dots in survey images. But when the team pointed the Dragonfly Telephoto Array at it, they instead saw a big blob.
Intrigued, they looked at it with several other instruments. They discovered that the galaxy is so sparse that it’s see-through, and its dots look like globular clusters, albeit strangely bright, similar to the Milky Way’s Omega Centauri cluster. They’re also about twice as large and squashed as globulars of similar brightness in our galaxy.
These ancient balls of stars loop around their parent galaxies with speeds determined by the galaxy’s total mass. Measure the globulars’ velocities, and you’ll measure the mass enclosed within their orbits — which usually turns up evidence for a whole lot of unseen matter.
Not this time. The 10 globular clusters the team clocked all move at most a third as fast as expected for galaxies of this mass (about 200 million Suns, or about a tenth that of the Small Magellanic Cloud). Their motions imply that the enclosed mass is roughly equal to the mass visible as stars.
In other words, there’s essentially no dark matter.
NGC 1052-DF2 is not the first tenuous galaxy to turn up. Astronomers have seen thousands of so-called ultra-diffuse galaxies (UDGs) since Van Dokkum, Roberto Abraham (University of Toronto), and their team first uncovered them with the Dragonfly array in 2014. But none of the others shows this paucity of dark matter.
How did something like this form? The team makes a couple of suggestions in the March 29th Nature, but nothing fits perfectly. For example, NGC 1052-DF2 could have formed from gas wrenched out of the nearby elliptical, which does show signs of a recent merger. But the UDG’s characteristics don’t match what astronomers would expect from that situation.
Another possibility is that DF2 has been stripped of its dark matter by the elliptical. Simulations by Alyson Brooks (Rutgers) and others indicate that when dwarfs pass through a big galaxy’s outskirts, they lose dark matter. “You can lose about 90% of the dark matter mass before any stars are stripped from the galaxy,” she says. That stripping is likely to cause the stars to “puff up,” she adds, perhaps creating an object like the diffuse, dark matter–less NGC 1052-DF2. But she’d like to know more about DF2’s trajectory before arguing the case.
Van Dokkum and his team don’t see signs of disturbance around the galaxy, which they think disfavors the flyby solution. It’s possible that any tidal features have faded away beyond detectability, but it’s unclear if there’s been enough time for that, Van Dokkum says.
That the coffee can exist without the cup indicates that the cup and coffee are both real, separate entities, the team concludes. If the presence of dark matter were only an illusion, arising because we’re using the wrong theory of gravity, then we’d always see signs of it in galaxies. But if the dark matter can sometimes be there and sometimes be absent, then dark matter exists.
Stacy McGaugh (Case Western Reserve University) isn’t so sure. He’s spent several years studying the strengths and weaknesses of modified Newtonian dynamics (MOND), a theory of gravity that does without dark matter by suggesting gravity works differently when low accelerations are involved. He agrees that NGC 1052-DF2’s globulars should move faster in MOND than they do. But he also points out that the galaxy is weird regardless of whether you accept dark matter’s existence or not.
“So yes, I find it troublesome for MOND,” he says. “But I don’t understand it any better in terms of dark matter.” Both dark matter and MOND have observational points in their favor, he explains, but he’s squeamish about upping or lowering the fraction of dark matter in any given galaxy just to make sense of stars’ motions — that’s an inference, not a prediction that can be proved or disproved, he says.
The Dragonfly team continues to look for more galaxies like NGC 1052-DF2. Of 23 other diffuse galaxies the astronomers are analyzing, three are potentially similar. With more than one galaxy in hand, astronomers might be able to say more about how these galaxies form and what they mean for dark matter.
Pieter van Dokkum et al. “A Galaxy Lacking Dark Matter.” Nature. March 29, 2018.
Pieter van Dokkum et al. “An Enigmatic Population of Luminous Globular Clusters in a Galaxy Lacking Dark Matter.” Posted to arXiv.org March 28, 2018.