Galaxies without dark matter confirmed

Two studies show visible matter and dark matter are not irrevocably tied together. Andrew Masterson reports.

Image from the Keck Observatory showing, in insets, the two galaxies that observations show lack dark matter.

van Dokkum et al

Two new studies have confirmed a contentious claim made in 2018 that some galaxies are completely devoid of dark matter.

The studies are both made by teams headed by Pieter van Dokkum, from Yale University, who was behind the initial assertion, made in March last year.

The claim concerned observations of a galaxy known as NGC 1052-DF2, which first came to van Dokkum’s attention the year before. Using the Keck Observatory in Hawaii, he and his colleagues noticed something peculiar about the way in which the globular clusters comprising the galaxy were moving.

They were travelling at speeds that suggested the total mass of the galaxy was equal to that only of its visible matter components. Dark matter – which is, as the name suggests, invisible – is known to comprise the vast majority of the mass in the universe. If it were present in NGC 1052-DF2, everything would have been moving much faster.

The conclusion was seemingly inescapable: the galaxy contained no dark matter at all – a condition held to be impossible.

The resulting paper, published in the journal Nature, was greeted with a mixture of astonishment, scepticism and, in some cases, anger.

“It was a little stressful at times,” say van Dokkum. “On one hand, this is how the scientific process is supposed to work; you see something interesting, other people disagree, you obtain new data, and in the end you learn more about the universe.

“On the other hand, although the majority of the critiques were constructive and polite, not all of them were. Every time a new critique came out, we had to scramble and figure out if we had missed something.”

The latest pair of papers provides strong evidence that they hadn’t.

The first, published in The Astrophysical Journal Letters, provides an updated and more detailed set of observations of NGC 1052-DF2, again using Keck. The results confirm that the velocities of the galaxy’s globular clusters are consistent with those expected from stellar mass alone.

“With this confirmation of the low velocity dispersion of NGC1052-DF2,” the researchers write, “the most urgent question is whether this ‘missing dark matter problem’ is unique to this galaxy or applies more widely.”

Appropriately enough, that exact question is answered by the second paper, published in the same journal.

NGC1052-DF2, it turns out, is not unique.

In the paper, van Dokkum and colleagues announce the discovery of a second galaxy that contains no dark matter. It rejoices in the name NGC 1052-DF4.

“Discovering a second galaxy with very little to no dark matter is just as exciting as the initial discovery of DF2,” he says.

“This means the chances of finding more of these galaxies are now higher than we previously thought. Since we have no good ideas for how these galaxies were formed, I hope these discoveries will encourage more scientists to work on this puzzle.”

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