Dark matter clue? Mysterious substance may be interacting with itself in nearby galaxy
The Milky Way satellite galaxy, named Crater II, may consist of self-interacting dark matter.
![telescope photo of a dozen satellite galaxies of our milky way, which shines in the background](https://cdn.mos.cms.futurecdn.net/BG43CcKKo7ocL2dspsz7DF-320-80.jpg)
A galaxy floating alongside our own some 380,000 light-years from Earth could offer fresh clues in the 90-year-long quest to determine the nature of dark matter, the invisible glue that holds galaxies together.
The mysterious substance accounts for over 80% of the universe's mass but has yet to be directly detected.
Scientists say the satellite galaxy, named Crater II, may consist of self-interacting dark matter (SIDM), which is a hypothetical variety of dark matter whose particles are predicted to interact via a hitherto unknown force beyond gravity. This hypothesis has in recent years gathered attention as an alternative form of conventional "cold" dark matter.
"When we started this project, we roughly knew how SIDM would work, but had no idea how well it would work in explaining the observations of Crater II," study co-author Hai-Bo Yu, who is a professor of physics and astronomy at the University of California, Riverside, told Space.com.
"Our computer simulations of Crater II analogs show that the agreement between [self-interacting dark matter] predictions and Crater II observations is surprisingly good, and the required strength of the dark matter self-interaction is larger than we initially anticipated."
Related: Exotic 'Einstein ring' suggests that mysterious dark matter interacts with itself
Discovered in 2016 in images taken by the Very Large Telescope in Chile, the Crater II galaxy is the fourth largest satellite of the Milky Way — after the large and small magellanic clouds and the Sagittarius galaxy. If it were visible to the naked eye, it would appear twice as large as the full moon, according to New Scientist. Crater II hosts a few billion old stars, which are sprinkled across 6,500 light-years, rendering the "feeble giant" remarkably faint — almost 100,000 times dimmer than the Milky Way.
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Despite multiple attempts over the years to simulate Crater II's properties, how the galaxy formed and maintains its relatively large size remains unclear. Astronomers know that Crater II evolves over eons under the gravitational influence of the Milky Way; our galaxy exerts a tidal force on it, which stretches its profile. Those tugs also influence its dark matter halo — a spherical, invisible structure surrounding Crater II — as well as the galaxy's stars.
"A useful analog is that the tidal force of the moon leads to ocean tides on the Earth," said Yu. "For the satellites of the Milky Way, the tidal force can strip away stars and dark matter, reducing the mass of the satellites over time."
However, recent measurements of the galaxy's orbit around the Milky Way suggest those interactions are too weak to explain Crater II's dark matter densities — that is, if dark matter is made of "cold," collisionless particles, as is predicted by the prevailing Lambda-CDM (LCDM or CDM) model of cosmology. Prolonged tidal interactions with the Milky Way should also have shrunk Crater II more than observed, scientists say.
Based on measurements of Crater II's orbit, Yu and other team members simulated the mass loss of stars and dark matter particles due to the Milky Way's tidal force. The team found that the galaxy's observed properties can be explained by dark matter particles that interact with each other.
Crucially, Crater II doesn't sport a high-density "cusp" of dark matter toward its center as predicted by the LCDM model. On the other hand, if dark matter were indeed made of self-interacting particles, collisions in the inner regions of a dark matter halo can transfer energy among the particles "and tend to make them carry the same amount of energy," said Yu. That would sort of even out Crater II's halo and explain its lack of a central cusp, according to the team's study, which was published this month in The Astrophysical Journal Letters.
SIDM also predicts that a galaxy will expand within the dark matter halo, which would explain Crater II's large size better than the CDM models, the researchers say.
"Our work shows that SIDM can well explain the unusual properties of Crater II, which challenges CDM," said Yu. "To further confirm whether dark matter indeed carries a new force, we hope to see more galaxies like Crater II."
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Sharmila Kuthunur is a Seattle-based science journalist covering astronomy, astrophysics and space exploration. Follow her on X @skuthunur.
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Unclear Engineer I am left wondering why the Crater II galaxy is "unusual" if its features are caused by "Self Interacting Dark Matter" and "dark matter" is supposedly everywhere and 6 times the amount of regular matter.Reply
Is the hypothesis that there are different kind of "dark matter" that cluster in different galaxies?
That seems like a weird concept for a cosmology based on the principle that the universe is the same everywhere, at least on a large scale.
Frankly, it seems like "cherry picking" the observations to support an hypothesis, rather than the discovery of some universal physics. -
Zdepthcharge
Throughout the 90s particulate dark matter seemed to be picking up a new superpower every time you turned around. Consistency is not a component of any particulate dark matter theory.Unclear Engineer said:Frankly, it seems like "cherry picking" the observations to support an hypothesis, rather than the discovery of some universal physics. -
Questioner "...its dark matter halo — a spherical, invisible structure surrounding Crater II ..."Reply
That geometry fits with a single, if tremendously huge mass source.
As if that galaxy’s center had a 'flat' mass curve source extending far outward from its central black hole. -
Torbjorn Larsson The Crater II was flagged as extreme, cold and dark with dark matter dominating normal matter with a factor 100:1 in the center and more at the edges. Despite that I don't see that the new paper tried to fit a cold dark matter model with the original paper suggestions of how an expanded dark matter halo can result:Reply
For example, star formation and subsequent supernova winds can give up gravitational energy to standard “cold” dark matter (CDM), lowering halo concentration as dark matter expands nonadiabatically (Pontzen & Governato 2014, and references therein).
So this single example of SIDS fit is dubious. Notably only one interaction strength gave a good fit, indicating lack of robustness, perhaps that model too would have benefitted from the above suggestions.
More generally, cold dark matter is evidenced in many different ways, so the new topic of extremely dilute dwarf galaxies is on the edge of an else well researched theory. And even if a particular SIDS would fit better, it would have to be tested on all those other cases.
Claims made without evidence can be dismissed without evidence.Zdepthcharge said:Throughout the 90s particulate dark matter seemed to be picking up a new superpower every time you turned around. Consistency is not a component of any particulate dark matter theory.
But as it happens, Cold Dark Matter models have been consistent for half a century:
The theory of cold dark matter was originally published in 1982 by James Peebles; ... In the cold dark matter theory, structure grows hierarchically, with small objects collapsing under their self-gravity first and merging in a continuous hierarchy to form larger and more massive objects. Predictions of the cold dark matter paradigm are in general agreement with observations of cosmological large-scale structure. ... Since the late 1980s or 1990s, most cosmologists favor the cold dark matter theory (specifically the modern Lambda-CDM model) as a description of how the universe went from a smooth initial state at early times (as shown by the cosmic microwave background radiation) to the lumpy distribution of galaxies and their clusters we see today—the large-scale structure of the universe. Dwarf galaxies are crucial to this theory, having been created by small-scale density fluctuations in the early universe; they have now become natural building blocks that form larger structures.
https://en.wikipedia.org/wiki/Cold_dark_matter