Richard A Lovett
In a study published in the journal Nature, scientists have found a galaxy that appears to contain no dark matter — the unknown material thought to be common in the universe because of its gravitational effect on normal matter.
It was a startling discovery, because galaxies similar to our own Milky Way generally appear to contain 30 times more of the mysterious substance than normal matter, while smaller galaxies can contain up to 400 times as much.
The dark-matter-free galaxy, called NGC 1052-DF2, lies 65 million light years away in the constellation Cetus. It initially caught the attention of astronomers because, while it’s about the size of the Milky Way, it contains only 0.5% as many stars.
“That makes it very diffuse,” says the study’s lead author, Pieter van Dokkum of Yale University, in Connecticut, US. “You can look straight through it. You can see galaxies behind it.”
It was discovered by a special, low-tech telescope in New Mexico called the Dragonfly Telephoto Array, which consists of a bundle of 400-millimetre camera lenses of the same type used by sports photographers, and can scan the sky for large, dim objects. So far, it’s found 23 of them, but NGC 1052-DF2 (the DF is for “Dragonfly”) stood out because it wasn’t just a big, diffuse blob.
“It also has a lot of dots on it, from very compact sources inside it,” van Dokkum says.
These dots proved to be globular clusters — bright balls of up to a million or more stars that extend from the centres of many galaxies like bejewelled halos. With help from bigger instruments including the Hubble Space Telescope and the 10-metre Keck telescope in Hawaii, van Dokkum’s team was able to get spectra from 10 of these clusters, looking for Doppler shifts that would reveal the speed at which they were moving.
These speeds, in turn, revealed the total amount of mass in the galaxy, which could then be compared to the amount of mass seen in its stars.
“We expected this galaxy to be very much dominated by dark matter,” van Dokkum says. “That would mean the velocities would be quite high [because] if there’s a lot of mass, you get high velocities.”
Instead, the velocities came up too low to be measured — somewhere below 10 kilometres per second, as opposed to the 30 van Dokkum’s team had expected. Since the normal matter in the galaxy would have produced a speed of eight kilometres per second, van Dokkum says, “there’s basically no room for dark matter. The maximum amount is about a factor of 400, at least, below what we thought.”
It was an unexpected, startling find. “It’s not that we were looking for galaxies without dark matter,” van Dokkum says. But it also has important implications, one of which is that it actually helps prove that dark matter really exists.
The fact that dark matter has, to date, been undetectable other than by its gravitational effects sometimes raises the question of whether it might actually be an illusion created by an error in our understanding of gravity — an error that makes it appear as though such a mystery substance exists.
But the discovery of NGC 1052-DF2 deals a serious blow to that theory. If “dark matter” were really nothing more than an artefact from a misunderstanding of gravity, it should appear to be present in all galaxies.
“Paradoxically, finding a galaxy without dark matter proves that it’s real,” says van Dokkum.
The find also raises questions about the standard model of galaxy formation.
Traditionally, van Dokkum says, it’s been thought that density whorls in the early universe’s distribution of dark matter were the key driver in creating the gravitational forces that attracted normal matter, which then condensed into stars and galaxies.
“The dark matter is the guide that tells the gas where to form stars,” he says.
But in the case of NCG 1052-DF2, that doesn’t seem to have happened. Why, van Dokkum says, is “completely unknown,” although he and his colleagues note that NGC 1052-DF2 is a companion to the giant elliptical galaxy NGC 1052, which may have played a role. Perhaps, they say, sometime in its turbulent history, NGC 1052 may somehow have ejected the matter that formed NGC 1052-DF2, while retaining the dark matter.
The next step, he says, is to look at more of the 23 objects found by Dragonfly and see if they too are devoid in dark matter.
“If [NGC 1052-DF2] is the only one with no dark matter, you can argue that something highly unusual happened to it,” he says. “But if there’s more than one, you have to find a more fundamental [explanation].”
Other scientists agree. “Finding more galaxies with low dark matter fractions like this would help settle that question,” says Katie Mack, an astrophysicist at North Carolina State University, US, who was not part of the study team.
Meanwhile, she says, the new find does indeed demonstrate that dark matter is a “truly separate kind of substance,” rather than being an illusion due to an error in our understanding of gravity. “Only by being a real, separate physical thing can dark matter be present in some cases and absent in others,” she explains.
But whatever its astrophysical ramifications, the new find probably doesn’t have much relevance to the effort to detect dark matter from our own galaxy as it sweeps past the Earth, says Benjamin Roberts, a dark matter researcher at the University of Reno, Nevada.
Currently, he says, astrophysicists think that our galaxy’s dark matter has a density of about one-third the mass of a hydrogen atom per cubic centimetre. That’s a figure important to people trying to build dark matter detectors, and the new finding, he says, is a useful reminder that it might not be accurate. But, he says, “It’s unlikely that the special circumstances that may have led this galaxy to have so little dark matter apply for the Milky Way.”
