“Lucky Find” by Astrophysicists Explains How Galaxies Can Exist Without Dark Matter

Beautiful Spiral Galaxy

In simulations, collisions cause smaller star groupings to lose material.

In a new Nature Astronomy study, an international team led by astrophysicists from the University of California, Irvine, and Pomona College report how, when tiny galaxies collide with bigger ones, the bigger galaxies can strip the smaller galaxies of their dark matter — matter that we can’t see directly, but which astrophysicists think must exist because, without its gravitational effects, they couldn’t explain things like the motions of a galaxy’s stars.

It’s a mechanism that has the potential to explain how galaxies might be able to exist without dark matter – something once thought impossible.

It started in 2018 when astrophysicists Shany Danieli and Pieter van Dokkum of Princeton University and Yale University observed two galaxies that seemed to exist without most of their dark matter.

“We were expecting large fractions of dark matter,” said Danieli, who’s a co-author on the latest study. “It was quite surprising, and a lot of luck, honestly.”

The lucky find, which van Dokkum and Danieli reported on in a Nature paper in 2018 and in an Astrophysical Journal Letters paper in 2020, threw the galaxies-need-dark-matter paradigm into turmoil, potentially upending what astrophysicists had come to see as a standard model for how galaxies work.

Dark Matter Distribution in Simulated Galaxy Group

Dark matter distribution in a simulated galaxy group, with brighter areas showing higher concentrations of dark matter. Circles show close-up images of the stellar light associated with two galaxies lacking dark matter. If these galaxies had dark matter, they would appear as bright regions in the main image. Credit:Morena et al.

“It’s been established for the last 40 years that galaxies have dark matter,” said Jorge Moreno, an astronomy professor at Pomona College, who’s the lead author of the new paper. “In particular, low-mass galaxies tend to have significantly higher dark matter fractions, making Danieli’s finding quite surprising. For many of us, this meant that our current understanding of how dark matter helps galaxies grow needed an urgent revision.”

The team ran computer models that simulated the evolution of a chunk of the universe – one about 60 million light years across – starting soon after the Big Bang and running all the way to the present.

The team found seven galaxies devoid of dark matter. After several collisions with neighboring galaxies 1,000-times more massive, they were stripped of most of their material, leaving behind nothing but stars and some residual dark matter.

“It was pure serendipity,” said Moreno. “The moment I made the first images, I shared them immediately with Danieli, and invited her to collaborate.”

Robert Feldmann, a professor at the University of Zurich who designed the new simulation, said that “this theoretical work shows that dark matter-deficient galaxies should be very common, especially in the vicinity of massive galaxies.”

UCI’s James Bullock, an astrophysicist who’s a world-renowned expert on low-mass galaxies, described how he and the team didn’t build their model just so they could create galaxies without dark matter – something he said makes the model stronger, because it wasn’t designed in any way to create the collisions that they eventually found. “We don’t presuppose the interactions,” said Bullock.

Confirming that galaxies lacking dark matter can be explained in a universe where there’s lots of dark matter is a sigh of relief for researchers like Bullock, whose career and everything he’s discovered therein hinges on dark matter being the thing that makes galaxies behave the way they do.

“The observation that there are dark matter-free galaxies has been a little bit worrying to me.” said Bullock. “We have a successful model, developed over decades of hard work, where most of the matter in the cosmos is dark. There is always the possibility that nature has been fooling us.”

But, Moreno said, “you don’t have to get rid of the standard dark matter paradigm.”

Now that astrophysicists know how a galaxy might lose its dark matter, Moreno and his collaborators hope the findings inspire researchers who look at the night sky to look for real-world massive galaxies they might be in the process of stripping dark matter away from smaller ones.

“It still doesn’t mean this model is right,” Bullock said. “A real test will be to see if these things exist with the frequency and general characteristics that match our predictions.”

As part of this new work, Moreno, who has indigenous roots, received permission from Cherokee leaders to name the seven dark matter-free galaxies found in their simulations in honor of the seven Cherokee clans: Bird, Blue, Deer, Long Hair, Paint, Wild Potato and Wolf.

“I feel a personal connection to these galaxies,” said Moreno, who added that, just as the more massive galaxies robbed the smaller galaxies of their dark matter, “many people of indigenous ancestry were stripped of our culture. But our core remains, and we are still thriving.”

Reference: “Galaxies lacking dark matter produced by close encounters in a cosmological simulation” by Jorge Moreno, Shany Danieli, James S. Bullock, Robert Feldmann, Philip F. Hopkins, Onur Çatmabacak, Alexander Gurvich, Alexandres Lazar, Courtney Klein, Cameron B. Hummels, Zachary Hafen, Francisco J. Mercado, Sijie Yu, Fangzhou Jiang, Coral Wheeler, Andrew Wetzel, Daniel Anglés-Alcázar, Michael Boylan-Kolchin, Eliot Quataert, Claude-André Faucher-Giguère and Dušan Kereš, 14 February 2022, Nature Astronomy.
DOI: 10.1038/s41550-021-01598-4

Funding for the work came from the National Science Foundation, sabbatical leave support for Moreno from Pomona College and the Harry and Grace Steele Foundation, and, for Danieli, from NASA through Hubble Fellowship grant HST-HF2-51454.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. Other collaborators include Francisco Mercado, Courtney Klein and Zachary Hafen, all from UCI.

4 Comments on "“Lucky Find” by Astrophysicists Explains How Galaxies Can Exist Without Dark Matter"

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  2. The unknown Genious | February 15, 2022 at 5:18 am | Reply

    “There is always the possibility that nature has been fooling us”
    Well I would say that this is far more likely than a possibility. If I understand this right, dark matter is hypothetical only, with it NEVER being measured or detected. If this is indeed the case, then it is not a fact that it exists, so to make assumptions based on something that is hypothetical, is asking for errors. If you assume, you make an (ass u me). I suggest you have it all wrong and that dark matter does not exist. To suggest you know the weight of the universe is preposterous and the entire theory of dark matter is base’s on this assumption. talk about asking to fail…

  3. Howard Jeffrey Bender, Ph.D. | February 15, 2022 at 1:39 pm | Reply

    Another possibility, from a view of String Theory, is that Dark Matter appears to us as an effect of string/anti-string annihilations. As you may know, quantum mechanics requires that strings must be formed as pairs in the quantum foam – a string and an anti-string – that immediately annihilate each other. Quantum mechanics also requires both the string and anti-string to be surrounded by “jitters” that reduce their monstrous vibrating energies. What if this jitter remains for a fraction of an instant after their string/anti-string annihilations? This temporary jitter would be seen by us as matter, via E=mc2, for that instant before it too returns to the foam. That’s why we never see it – the “mass” lasts only for that instant but is repeated over and over and over, all over. Specifics on this can be found in my YouTube, Dark Matter – A String Theory Way at https://www.youtube.com/watch?v=N84yISQvGCk

  4. Very Very Interesing.

    On a lighter note, I think Reading Comics should be made Compulsoory in High School, especially for the STEM stream! Do you remember the stories of the Big Bully with with huge muscles walking on the beach, and kicking sand on te face of the little guy lying on the beach , under the multicoloured beach umbrella with his beautiful girfrien,d and forcefull dragging /taking her away. The Pretty Girl Could be any Colour. The Colour of the skin is a result of one chromosome, as we all already knoow from Biological Sciences.

    On a Serious note it is great to know Glaxies exist without a Black Hole . I am confident that free floating Black Holes also exist without a a Galaxy.

    1. Normal : Low Mass Galaxies have a greater percentage of Dark Matter!
    New Normal. Galaxies can exist without Dark Matter.

    2. Evolution of the Universe with 60 million Light Years across assumption used for the simulations. Can “What If” scenarios be done under different assumptions, for the “life” of the Universe, after the Big Bang .

    3. The Big Bang is itself an assumption. If we can run the simulation with with the assumption of such an event missing, and under the assumption that time itself need not be linear and could be non-linear or cyclical as can be space and matter (a form of energy) , we may get interesting results as far as the components of matter, Space-Time and its various components as obseved by Astronomers. Are we being “Flat-Earthers” on a Universe Scale?

    4. It is an interesting Find that so far we have only found seven such galaxies with their Black Holes taken away by the neihbouring Massive Galaxies 1000 times their size. Let us see if WEBB will help us find many more.

    5. Has anyone explored the possibility that a lack of a black hole at the centre of such galaxies is only one possibility? There could be Galaxies with a non-black-hole variant floating around in the Universe, including all the colours of the rainbow, we get from white light , including a white hole variant, which if not detected yet in this universe could be found in others in the multiverse.

    6. Even if such a galaxy with the Black Hole missing eists in line with the general characteristics and frequency as predicted by models sir, it doesnt prove that Mother Nature is not fooling you! We need to rethink the Universe taking into consideration all that has been outlined in Points (1) to (5) above.

    The truth is, we are really clueless about how we, the world, the universe and the multiverse (if it exists) came into existence or if any of its is real!

    These ladies , they always give us the raspberry and thumb their nose at us. Regardless “Happy Belated Valentines Day ” Mother Nature.

    Views expressed are personal and not binding on anyone.

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