How galaxies can exist without dark matter
In simulations, collisions cause smaller star groupings to lose material


Date:
February 14, 2022
Source:
University of California - Irvine
Summary:
Astrophysicists 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.



FULL STORY ========================================================================== [Galaxy illustration | Credit: (c) alphaspirit / stock.adobe.com] Galaxy illustration (stock image).

Credit: (c) alphaspirit / stock.adobe.com [Galaxy illustration | Credit:
(c) alphaspirit / stock.adobe.com] Galaxy illustration (stock image).

Credit: (c) alphaspirit / stock.adobe.com Close 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.

"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 an 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." 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.

========================================================================== Story Source: Materials provided by
University_of_California_-_Irvine. Note: Content may be edited for style
and length.


========================================================================== Related Multimedia:
* Dark_matter_distribution_in_a_simulated_galaxy_group ========================================================================== Journal Reference:
1. Moreno, J., Danieli, S., Bullock, J.S. et al. Galaxies lacking dark
matter produced by close encounters in a cosmological
simulation. Nat Astron, 2022 DOI: 10.1038/s41550-021-01598-4 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220214121243.htm

--- up 10 weeks, 2 days, 7 hours, 13 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1:317/3)