Science Gazette

How can galaxies live in the absence of dark matter?

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Smaller star groups lose material as a result of collisions in simulations.

Astrophysicists have discovered that when small galaxies collide with larger ones, the larger galaxies can strip the smaller galaxies of their dark matter — matter that we can’t see directly but that astrophysicists believe must exist because they couldn’t explain things like the motions of a galaxy’s stars without its gravitational effects.

An international team led by astrophysicists from the University of California, Irvine, and Pomona College report in Nature Astronomy that when small galaxies collide with larger ones, the larger galaxies can strip the smaller galaxies of their dark matter — matter that we can’t see directly but that astrophysicists believe must exist because they couldn’t explain things like the motions of a galaxy’s stars without its gravitational effects.

It’s a method that might explain how galaxies could survive without dark matter, which was previously considered to be impossible.

It began in 2018, when Princeton University and Yale University astrophysicists Shany Danieli and Pieter van Dokkum discovered two galaxies that seemed to live without most of their dark matter.

“We were anticipating substantial proportions of dark matter,” said Danieli, one of the study’s co-authors. “It was extremely unexpected, and a lot of luck, to be honest.”

The fortuitous discovery, which van Dokkum and Danieli published in Nature in 2018 and Astrophysical Journal Letters in 2020, threw the galaxies-need-dark-matter paradigm into disarray, possibly upending what astrophysicists had come to regard as a conventional explanation for how galaxies behave.

“For the previous 40 years, it’s been known that galaxies contain dark matter,” said Jorge Moreno, an astronomy professor at Pomona College and the paper’s primary author. “Low-mass galaxies, in particular, have considerably greater dark matter percentages, which makes Danieli’s discovery extremely startling. For many of us, this meant that we needed to revise our existing understanding of how dark matter aids galaxies’ growth.”

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The researchers used computer models to replicate the history of a section of the universe, measuring roughly 60 million light years wide, from just after the Big Bang to the present day.

Seven galaxies were discovered to be dark matter-free. They were stripped of most of their material after repeated collisions with nearby galaxies 1,000 times more massive, leaving just stars and some remnant dark matter.

“It was sheer coincidence,” Moreno said. “I quickly shared the initial photographs with Danieli and encouraged her to contribute as soon as I finished them.”

“This theoretical study implies that dark matter-deficient galaxies should be relatively frequent, particularly in the proximity of big galaxies,” said Robert Feldmann, a professor at the University of Zurich who devised the new simulation.

James Bullock, an astronomer at UCI who specializes in low-mass galaxies, explained how he and his colleagues didn’t construct their model just to generate galaxies without dark matter, which he believes makes the model stronger since it wasn’t planned to produce the collisions they discovered. Bullock said, “We don’t presume the relationships.”

Confirming that galaxies without dark matter can be explained in a world with plenty of dark matter is a relief for researchers like Bullock, whose career and everything he’s found depends on dark matter being the force that causes galaxies to behave how they do.

“The discovery of dark matter-free galaxies has been a little concerning to me,” Bullock remarked. “We have a successful model, based on decades of hard effort, in which the majority of matter in the universe is dark. It’s always possible that nature has been deceiving us.”

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But, Moreno argued, “you don’t have to get rid of the traditional dark matter model.”

Moreno and his team believe that now that astrophysicists understand how a galaxy loses its dark matter, they will encourage researchers to explore for real-world big galaxies that are in the process of pulling dark matter away from smaller ones.

“That doesn’t rule out the possibility that this model is incorrect,” Bullock added. “A true test will be to check whether these entities exist in the frequency and general features that our projections anticipate.”

Moreno, who has Cherokee ancestors, was granted permission by Cherokee elders to name the seven dark matter-free galaxies discovered in their simulations after the seven Cherokee clans: Bird, Blue, Deer, Long Hair, Paint, Wild Potato, and Wolf.

“”I have a personal connection to these galaxies,” Moreno remarked, adding that “many individuals of indigenous origin were stripped of our culture, much as the more massive galaxies took the smaller galaxies of their dark matter.” Our core, however, has not changed, and we are still prospering.”

The National Science Foundation provided funding for the project, as well as sabbatical leave support from Pomona College and the Harry and Grace Steele Foundation for Moreno and NASA through Hubble Fellowship grant HST-HF2-51454.001-A awarded by the Space Telescope Science Institute, which is run by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555 for Danieli. Francisco Mercado, Courtney Klein, and Zachary Hafen, all of UCI, are among the other contributors.

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