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
technique that could explain how galaxies could exist without dark matter,
which was previously thought impossible.
It began
in 2018, when Princeton University and Yale University astrophysicists Shany
Danieli and Pieter van Dokkum discovered two galaxies that appeared 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 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, potentially upending what astrophysicists had come to
regard as a standard explanation for how galaxies behave.
"For
the previous 40 years, it's been known that galaxies have dark matter,"
said Jorge Moreno, an astronomy professor at Pomona College and the paper's
lead author. "Low-mass galaxies, in particular, have considerably higher
dark matter fractions, 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."
The
researchers used computer models to mimic the evolution of a 60 million
light-year-wide slice of the cosmos from shortly 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 neighbouring galaxies 1,000 times
more massive, leaving only stars and some remnant dark matter.
"It
was pure serendipity," Moreno explained. "I quickly shared the first
photographs with Danieli and invited her to collaborate as soon as I finished
them."
"This
theoretical work implies that dark matter-deficient galaxies should be
relatively common, especially in the proximity of big galaxies," said
Robert Feldmann, a professor at the University of Zurich who devised the new
simulation.
UCI
scientist James Bullock, a world-renowned expert on low-mass galaxies,
explained how he and his colleagues didn't build their model just to create
galaxies without dark matter, which he believes makes the model stronger
because it wasn't planned to produce the collisions they discovered. Bullock
explained, "We don't presuppose the interactions."
Confirming
that galaxies without dark matter can be explained in a universe with plenty of
dark matter is a relief for researchers like Bullock, whose career and
everything he's discovered 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."
"You
don't have to get rid of the normal dark matter concept," Moreno added.
Moreno and
his team hope 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 if 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
thriving."
References:
- Jorge Moreno, Galaxies lacking dark matter produced by close encounters in a cosmological simulation, Nature Astronomy (2022). DOI:10.1038/s41550-021-01598-4. www.nature.com/articles/s41550-021-01598-4
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