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Image of Keck observatory showing, in insets, the two galaxies that observations reveal without dark matter
van Dokkum et al
Two new studies have confirmed a controversial claim made in 2018 that some galaxies are completely devoid of dark matter.
The studies are carried out by teams led by Pieter van Dokkum, from Yale University, behind the initial statement made last March.
The statement concerned observations of a galaxy known as NGC 1052-DF2, which first attracted van Dokkum's attention the previous year. Using the Keck Observatory in Hawaii, he and his colleagues noticed a peculiarity in the way the globular groups of the galaxy were moving.
They were traveling at speeds suggesting that the total mass of the galaxy was equal to that of its visible-only components. Dark matter – which is, as its name implies, invisible – is known to constitute the vast majority of the mass of the universe. If it was present in NGC 1052-DF2, everything would have been much faster.
The conclusion was apparently inescapable: the galaxy contained no dark matter, a condition considered impossible.
The resulting paper, published in the journal Nature, was greeted with a mixture of astonishment, skepticism and, in some cases, anger.
"It was a little stressful at times," says van Dokkum. "On the one hand, here's how the scientific process is supposed to work. you see something interesting, others do not agree, you get new data and you learn more about the universe.
"On the other hand, although the majority of critics are constructive and polite, they have not all been. With each new criticism, we had to try to find out if we had missed something. "
The last pair of articles clearly shows that this was not the case.
The first, published in The letters of the astrophysical journal, provides an updated and more detailed set of observations of NGC 1052-DF2, again with the help of Keck. The results confirm that the globular cluster velocities of the galaxy correspond to those expected from the star mass alone.
"With this confirmation of the low-speed dispersion of NGC1052-DF2, the researchers write," the most urgent question is whether this "missing dark matter problem" is unique to this galaxy or more broadly applicable. "
Appropriately, the second article, published in the same newspaper, answers this specific question.
It turns out that NGC1052-DF2 is not unique.
In the newspaper, van Dokkum and his colleagues announce the discovery of a second galaxy that does not contain dark matter. He is looking forward to the name NGC 1052-DF4.
"Discovering a second galaxy with very little or no dark matter is just as exciting as the initial discovery of DF2," he says.
"This means that the chances of finding more of these galaxies are now higher than expected. As we do not have good ideas on the formation of these galaxies, I hope that these discoveries will encourage more scientists to work on this puzzle. "
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