Picture: NASA, ESA, STScI, Zili Shen (Yale), Pieter van Dokkum (Yale), Shany Danieli (IAS), (Image Processing) Alyssa Pagan (STScI)
Scientists discover how galaxies can exist without dark matter
In 2018 astrophysicists discovered galaxies that seem to lack most of their dark matter — matter that we can’t see directly, but which is needed to explain the observed speed of stars in most galaxies. This surprising result threw the galaxies-need-dark-matter paradigm into turmoil, potentially upending what astrophysicists had come to see as a standard model for how galaxies behave.
In a new Nature Astronomy paper, though, an international team involving researchers from the Institute of Computational Science at the University of Zurich, report how bigger galaxies can strip smaller galaxies of their dark matter while leaving the galaxies in a stable state — a mechanism that has the potential to explain the original discoveries.
“It’s been established for the last 40 years that galaxies have dark matter,” said Dr. Jorge Moreno (Pomona College), the lead author of the new study. “In particular, low-mass galaxies tend to have significantly higher dark matter fractions. For many of us, this meant that our current understanding of how dark matter helps galaxies grow needed an urgent revision.”
The team analyzed 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 day. Dr. Robert Feldmann (University of Zurich), who ran the simulation on which this current study is based, added that “this theoretical work suggests that dark matter deficient galaxies are quite common, especially in the proximity of massive galaxies.”
Dr. James Bullock, an astrophysicist at UC Irvine and expert on low mass galaxies, described how the team did not build their model specifically so they could create galaxies without dark matter — something he said makes the model stronger because it was not designed in any way to create the collisions they eventually found. “We don’t presuppose the interactions,” said Bullock.
In the simulation, seven galaxies devoid of dark matter were found. These seven galaxies were originally dark-matter rich. But, after several collisions with neighboring galaxies 1,000-times more massive than them, they were stripped of most of their material, leaving behind nothing but stars and some residual dark matter.
Now that astrophysicists know how a galaxy may lose its dark matter, the team hopes to inspire a search for real-world massive galaxies that may be in the process of stripping dark matter away from smaller ones. “Our model makes specific predictions for the properties and abundance of dark matter poor galaxies. Testing whether these predictions agree with observations will be a critical next step” said Feldmann.
Jorge Moreno, Shany Danieli, James S. Bullock, Robert Feldmann, Philip F. Hopkins, Onur Catmabacak, Alexander Gurvich, Alexandres Lazar, Courtney Klein, Cameron B. Hummels, Zachary Hafen, Francisco J. Mercado, Sijie Yu, Fangzhou Jiang, Coral Wheeler, Andrew Wetzel, Daniel Angles-Alcazar, Michael Boylan-Kolchin, Eliot Quataert, Claude-Andre Faucher-Giguere, Dusan Keres, Galaxies lacking dark matter explained by a state-of-theart cosmological simulation, Nature Astronomy, https://doi.org/10.48550/arXiv.2202.05836
University of Zurich
Institute for Computational Science
Prof. Robert Feldmann
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