Astronomers have discovered that a satellite of the Milky Way has left a “wake” in the dark matter halo that surrounds our galaxy.

A hefty galactic dwarf is falling toward the Milky Way. Known as the Large Magellanic Cloud, it’s plowing through — and pulling on — the fog of dark matter that surrounds our galaxy. Sparse stars float like the tips of dark matter icebergs in this galactic ocean. Now, astronomers have definitively detected a pile-up in those stars (and presumably the dark matter, too) that this dwarf has left in its wake. The results appear in the April 22nd Nature.

The Large Magellanic Cloud's "wake"
Images of the Milky Way and the Large Magellanic Cloud (LMC) are overlaid on a map of the halo that surrounds our galaxy (shown in shades of blue). The smaller blob below the galactic plane is a region thick with halo stars, left behind in the LMC's wake as the dwarf passed through the area. The larger feature above the galactic plane also corresponds to a higher stellar density, created by the shift in the center of mass in the Milky Way–LMC system.
NASA / ESA / JPL-Caltech / Conroy et. al. 2021

Charlie Conroy (Center for Astrophysics, Harvard & Smithsonian) and his colleagues plotted the distances and positions of 1,301 giant stars in our galaxy’s outermost reaches, between 200,000 and 330,000 light-years from the galactic center. The European Space Agency’s Gaia satellite provided these stars’ positions on the sky, while data from NASA’s Widefield Infrared Survey Explorer mission provided their distances.

Two regions on the sky stand out for their thickness of halo stars, one above the galactic plane and one below. The latter extends toward the Large Magellanic Cloud in the south; the denser region of stars in the north is even larger, spanning a quarter of the sky.

The team compared the observed stellar densities to computer simulations that trace the gravitational effects of the Large Magellanic Cloud as it falls in toward the Milky Way for the first time. Nicolás Garavito-Camargo (University of Arizona) led the work on the simulations, which show that the dwarf galaxy should create a wake behind it as it falls through our galaxy’s halo, trailing stars (and dark matter) behind it. The expected wake matches the dense region below the plane.

“We think this wake is made up of dark matter,” says Garavito-Camargo. “It drags stars along with it, which is how we can detect it.”

If the Large Magellanic Cloud were looping around our galaxy for a second or third time, any such wake would be smeared out; the fact that we see it so clearly provides strong evidence that the dwarf is making its first pass around the Milky Way.

Meanwhile, the Milky Way itself moves in response to the massive dwarf, which has a tenth our galaxy’s heft. As the pair’s center of mass shifts, a weaker but wider-ranging effect gathers stars closer together in a large region above the galactic plane.

“This is a very cool result,” says Geraint Lewis (University of Sydney), who was not involved in the study. “This gravitational wake behind a large dwarf galaxy was predicted a while ago, and it was expected that stars should pile up also, but given that the halo is so sparse it has taken a lot of work to identify the signature in the stars.”

Both above and below the plane, the simulations reproduce reality quite well, especially when you consider that they weren’t calibrated or fixed to the observations in any way. The main difference is that, in real life, the stellar crowding is actually more pronounced than it was in the simulations. This could be because of complicating factors in reality that are not yet accounted for in the simulations, such as the existence of the less massive Small Magellanic Cloud.

There’s also still a lot that we don’t know about our galaxy’s dark matter halo. Results like this one will help astronomers refine their understanding of the shape it takes around our galaxy.

“As we get more data, it is clear that dark matter halo is not a simple structure and our calculation of the properties of the halo will be more complex,” Lewis adds. “The existence of a wake of dark matter does offer some interesting possibilities in unraveling its nature.”

The wake’s discovery, for example, points to higher-density regions of dark matter, where particles may annihilate more frequently (but still rarely), betraying their existence by the photons they emit. Such regions might become targets of future dark matter searches. The theorists of the group are also working to fine-tune the simulations, turning knobs to see what dark matter scenarios best match observations.

"What has been a purely theoretical prediction has now been validated by observational data, providing a compelling argument for the existence of dark matter," says study coauthor Gurtina Besla (University of Arizona).


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Comments


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Peter Wilson

April 28, 2021 at 8:14 pm

"This could be because of complicating factors in reality that are not yet accounted for..."

That sounds reasonable.

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Andrew James

April 29, 2021 at 4:33 am

Wow. Nice cherry picking. The fuller quote in context is: "This could be because of complicating factors in reality that are not yet accounted for in the simulations..." It is a bit disingenuous to infer the "complicating factors" are discount dark matter when the researchers are actually referring to other gravitational potentials like the SMC that may create wakes that were not not included in their investigations.

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Peter Wilson

April 29, 2021 at 8:52 am

Whoa! Whoa! Whoa…

It is not the least bit disingenuous to infer that "complicating factors" may account for other enigmas: it’s the scientific method. Whenever a simple model fails, the addition of complicating factors is the default scientific response.

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Andrew James

April 29, 2021 at 10:19 pm

Eh? What model? This paper finds observational evidence based on the motion of 1300 giant stars that there are additional gravitational forces in the wake of the LMC that supports the idea of the existence of dark matter. Yet amazingly you think this now is an example of model failure! They predicted nothing to do with theory!
Worst, their computer simulations here didn't even factor in a dark matter component, but do only measured the effects of LMC's gravity on the motion of these stars and found an anomaly. The only "complicating factors" they are concerned with are additional influences from other potential gravitational sources like the SMC.
Yet you also previously stated weeks ago; "Because until some realism is added to the model, the brouhaha over dark energy is nonsense." But now when actual observational and supportive evidence becomes available that contradicts your beliefs, you blame science and the scientific method: then pick out anything that diminishes, dismisses or trivialises the research's conclusions.
Fact. Dark matter has some good observational support for its existence, but we still don't understand how in manifests itself. Saying it doesn't exist without any substantive proof or evidence is just pure folly. Thanks.

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Andrew James

April 30, 2021 at 2:58 am

Quote from Vargya et al. (2021) : "The cold dark matter (CDM) plus dark energy ΛCDM cosmological model has been the most successful model for understanding the large-scale structure of the Universe." See [1]. It is clear this research helps to understand the interaction of with shape of the main dark matter (DM) halo. Interesting discovery.

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Peter Wilson

May 3, 2021 at 2:51 pm

Dark matter? “Dr. Becky” recently did a podcast on “complicating factors” not yet accounted for: General Relativity and the rest of the universe. Spiral galaxies are modeled as if spinning in an empty universe, using Newtonian gravity. It is hard to imagine these complicating factors—GR and the rest of the universe—could account for dark matter, but until they are factored in, we don’t know.

And lambda? Ditto. The simplest model, using only density and expansion rate, failed to account for the observed acceleration. But instead of adding “complicating factors,” modelers added a mystery, lambda. That is not the scientific method. Until these “complicating factors” are factored in—action (LD), reaction (η, “eta”) and distance between (R_i)—the addition of mystery to the model is not to be taken seriously.

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Andrew James

May 3, 2021 at 8:11 pm

Just laughable. Smethurst has said no such thing and it looks like you've again cherry picked bits that just suits your agenda. Worst none of this is relevant to this story.
That you are unable to distinguish context of two words 'complicating factors' used here is worrying. The article quotes: "The main difference is that, in real life, the stellar crowding is actually more pronounced than it was in the simulations. This could be because of complicating factors in reality that are not yet accounted for in the simulations, such as the existence of the less massive Small Magellanic Cloud." The clear indication of some real observation effect not of theory.
As for saying; "Spiral galaxies are modeled as if spinning in an empty universe, using Newtonian gravity.", clearly isn't true, because Modified Newtonian Dynamics (MOND) is often adopted as a 'factor.' e.g. It accounts for the observed rotation curves of galaxies or the properties of the Local Group of galaxies. A demonstration of the success of MOND in the Milky Way is here [A] Your statement here s just dead wrong. How do you account for the rotation curve anomalies then?
Also "modelers added a mystery, lambda" Nope. Actually, Einstein did!

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Andrew James

May 3, 2021 at 11:18 pm

"Until these “complicating factors” are factored in—action (LD), reaction (η, “eta”) and distance between (R_i)—the addition of mystery to the model is not to be taken seriously."

OK. Please provide a peer reviewed cosmological paper that supports such unsupported notions.
Also you've been writing this kind of guff for +10 years now. e.g. In 2012 you wrote: "Cosmologist have over-simplified their model of the cosmos, hanging everything on just two parameters, density and expansion rate." Like then this isn't true is it? e.g. What about the quantum mechanical effects just as an example. Where does the energy for the expansion come from?

Note: Yes, I've read your fringe 2014 vixra paper "Causality and Duality in Cosmology." It makes little to no sense, too simplistic, and ignores swathes of even basic physics. Really. Does anyone seriously still think that Newtonian dynamics is adequate enough to explain the whole cosmological nature of the Universe? That there are zero cites and no comments of support, also means "Until these “complicating factors” are factored in—action (LD), reaction (η, “eta”) and distance between (R_i)—the addition of mystery to the model is not to be taken seriously."

OK. Please provide a peer reviewed cosmological paper that supports such unsupported notions.
Also you've been writing this kind of guff for +10 years now. e.g. In 2012 you wrote: "Cosmologist have over-simplified their model of the cosmos, hanging everything on just two parameters, density and expansion rate." Like then this isn't true is it? e.g. What about the quantum mechanical effects just as an example.
Note: Yes, I've read your 2014 vixra paper "Causality and Duality in Cosmology." It makes little to no sense, too simplistic, and ignores swathes of even basic physics. Really. Does anyone seriously still think that Newtonian dynamics is adequate enough to explain the whole cosmological nature of the Universe? One huge flaw in your argument a binary star, is that spiraling/decreasing orbit is caused by mass loss via winds as the stars evolve. That there are zero cites, no comments of support, means your model can't be taken seriously either.

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Andrew James

May 3, 2021 at 11:41 pm

CORRECTION</b
"Until these “complicating factors” are factored in—action (LD), reaction (η, “eta”) and distance between (R_i)—the addition of mystery to the model is not to be taken seriously."

OK. Please provide a peer reviewed cosmological paper that supports such unsupported notions.

Also you've been writing this kind of guff for +10 years now. e.g. In 2012 you wrote: "Cosmologist[s] have over-simplified their model of the cosmos, hanging everything on just two parameters, density and expansion rate." Like then this isn't true is it? e.g. What about the quantum mechanical effects just as an example. Where does the energy for the expansion come from?

Note: Yes, I've read your 2014 vixra paper "Causality and Duality in Cosmology." It makes little to no sense, too simplistic, and ignores swathes of even basic physics. Really. Does anyone seriously still think that Newtonian dynamics is adequate enough to explain the whole cosmological nature of the Universe? One huge flaw in your argument a binary star, is that spiraling/decreasing orbit is caused by mass loss via winds as the stars evolve. That there are zero cites, no comments of support, means your model can't be taken seriously either.

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Peter Wilson

May 5, 2021 at 2:23 pm

You’ve done some research. Thanks.

If there were a published paper I could site, agreeing with my thesis, that the standard model needs to include action and reaction, η and R_i, then my point would have already been taken! I submitted the paper to a journal; they must have posted it to arXive. They said they couldn’t even read it, without sponsorship of a published PhD. But nobody wants to be wrong, so the published PhDs wont touch it. The doors to the journals are closed to critique from outsiders.

I don’t see any way forward, except to include “reaction” in the model, like pressure. Pressure can be safely ignored...because we have an equation for it. Maybe you are right! Maybe the reaction term can be ignored, just like the pressure term is. But without an equation for the reaction term, we just don’t know.

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Andrew James

May 6, 2021 at 2:32 am

I've done nothing other than read your own claims. The failure is not that 'nobody wants to be wrong, so the published PhDs won't touch it' at all, it is mostly that your assumptions are just wrong or misunderstood. arVix works by sponsorship not censorship, and no amount of moaning about being ignored holds sway. In its present form, no one would likely formally review it, and even if it were critiqued, it would be simply rejected from any formal publication. If you don't understand how scientfic publications and peer review works, citations, proofs or how to test ideas by predictions; then your notions will never get off the ground.

Your own incorrect and false statement; "Cosmologist[s] have over-simplified their model of the cosmos, hanging everything on just two parameters, density and expansion rate." is enough to completely reject what you say. e.g. They don't.

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Andrew James

May 6, 2021 at 3:26 am

Eh."...they must have posted it to arXiv." No. It appears on viXra not arXiv. viXra is an open repository for new scientific articles unrelated to Cornall's arXiv. Evidence says you submitted there. It was never on arXiv at all.

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