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Galaxies without dark matter confuse astronomers

This Hubble image captures a series of galaxies that are unusual in that they appear to have no dark matter.Credit: NASA/ESA/P. van Dokkum, Yale Univ.

Astronomers believe that galaxies cannot form without the gravitational pull of dark matter. So a trail of galaxies free of this mysterious material, with no apparent cause, would be a remarkable find. In a paper published in Nature on May 181, astronomers say they’ve observed such a system — a line of 11 galaxies that don’t contain dark matter, all of which could have formed from the same ancient collision. But many of their colleagues aren’t convinced that the claim is much more than a hypothesis.

This kind of system could be used to learn more about the formation of galaxies and the nature of dark matter itself. “If proven right, this could certainly be exciting for galaxy formation. However, the jury is out yet,” said Chervin Laporte, an astronomer at the University of Barcelona in Spain.

The finding focuses on two galaxies described by Pieter van Dokkum at Yale University in New Haven, Connecticut, and his co-authors in 2018 and 2019. Their stars moved so slowly that the gravitational pull of dark matter was not needed to orbit their orbits. explain, so the team concluded that the galaxies do not contain dark matter. The finding was controversial because the galaxies, named DF2 and DF4, appeared stable and differed from the only other known dark matter-free galaxies, which are new and short-lived, created in the arms of larger galaxies whose dark matter is removed by a neighbour. How DF2 and DF4 came to be was a mystery.

Telltale trail

In the latest paper, Van Dokkum’s team not only connects the two unusual galaxies, but says their properties are consistent with the fact that they formed during a rapid collision eight billion years ago, which also spawned more such structures. “This single statement explains so many strange things about these galaxies,” says Van Dokkum.

The team borrowed its scenario from simulations originally created to explain unique features in large-scale collisions between galaxy clusters. The researchers suggest that when two ancestor galaxies collided head-on, their dark matter and stars would have passed each other; the dark matter would not have interacted and the stars would have been too far apart to collide. But as the dark matter and stars raced along, gas in the space between the stars of the two galaxies would have crashed, compressed and slowed down together, leaving a trail of matter that later formed new galaxies without dark matter.

Next, the researchers looked for such galaxies in the line between DF2 and DF4. They identified three to seven new candidates for dark matter-free galaxies, as well as strange, faint galaxies at either end, which could be the dark matter and stars left over from the precursor galaxies. “It was staring you in the face once you knew what to look for,” says van Dokkum.

If true, this image could help astronomers understand how dark matter behaves and learn about the conditions under which galaxies can form. Such a galactic collision could also be used as a “new laboratory” to understand whether dark matter interacts with itself, said Go Ogiya, an astronomer at Zhejiang University in Hangzhou, China.

Open questions

While plausible, Van Dokkum’s model describes only one of a number of ways these galaxies could have been created, says Priyamvada Natarajan, an astrophysicist at Yale who is not a member of Van Dokkum’s team. But it’s intriguing and, crucially, makes testable predictions, she says.

Measurements of the precise distances and velocities of candidate galaxies could prove that they are part of the same series and not coincidentally along the same line of sight, says Michelle Collins, an astronomer at the University of Surrey in Guildford, UK. “For me it’s a big open question whether this is a real rule or not.”

Astronomers also need to measure the masses of the phantom galaxies at the ends of the line — the potential precursors — to test whether they contain a lot of dark matter, as the model predicts, Laporte adds.

Others wonder if an exotic explanation is needed. Ignacio Trujillo, an astronomer at the Institute of Astrophysics of the Canary Islands in La Laguna, Spain, leads a team that proposes that DF2 and DF4 are closer to Earth than Van Dokkum’s measurements suggest, and therefore contain more dark matter. than it seemed at first glance.

Astronomers also need to see a reliable simulation that shows that the scenario Dokkum’s team is describing is plausible, said Mireia Montes, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland. “For now, there are many assumptions, but none of the simulations support them,” she says.

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