Space

Astronomers find galaxy with no dark matter – and why that really matters

A snap of the galaxy NGC1052–DF2, taken with the Hubble Space Telescope
A snap of the galaxy NGC1052–DF2, taken with the Hubble Space Telescope
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The Dragonfly Telephoto Array is good at spotting dim objects that other telescopes might miss
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The Dragonfly Telephoto Array is good at spotting dim objects that other telescopes might miss
The Dragonfly Telephoto Array was used to identify the strange galaxy, before follow-up observations were made using Hubble and Keck
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The Dragonfly Telephoto Array was used to identify the strange galaxy, before follow-up observations were made using Hubble and Keck
A snap of the galaxy NGC1052–DF2, taken with the Hubble Space Telescope
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A snap of the galaxy NGC1052–DF2, taken with the Hubble Space Telescope

Dark matter is invisible and undetectable – which begs the question, how do we know it's there at all? Astronomers first suspected there was more matter than meets the eye in the 1930s, when it was observed that galaxies were moving faster than their visible mass should allow. But now there's a new wrinkle to the story, as scientists at Yale University have for the first time discovered a galaxy that appears to have no dark matter at all. Ironically, the find may just turn out to be the exception that proves the rule.

Gravity is a product of mass, so the speed at which stars and galaxies move should be proportional to their mass. But during observations of a cluster of galaxies in 1933, Swiss astrophysicist Fritz Zwicky found that wasn't the case, and calculated that there had to be far more matter in the cluster than they could see. He dubbed this mysteriously missing mass "dark matter," and scientists have been hunting for it ever since.

But that hunt has been made tricky by the fact that dark matter doesn't reflect or absorb light, and can only be indirectly detected through its gravitational interactions with ordinary (baryonic) matter. Further studies over the past 85 years have shed more light on dark matter, suggesting it clings to planets as "hairs," fills galaxies in a fuzzy, excited form, shapes the structure of galaxy clusters and connects them to each other in filaments.

Across the universe, dark matter far outweighs the normal stuff. In galaxies similar to our own Milky Way, that ratio is usually around 30 to 1, while dwarf galaxies may have up to 400 times more dark matter than ordinary matter.

That makes the galaxy NGC1052–DF2 extremely odd – it seems to have no dark matter at all. In the new study, Yale researchers measured the movements of 10 star clusters in the galaxy, and found that they were moving at about 23,000 mph (37,000 km/h). That's relatively slow, meaning that there's no "missing" mass.

"If there is any dark matter at all, it's very little," says Pieter van Dokkum, lead author of the study. "The stars in the galaxy can account for all the mass, and there doesn't seem to be any room for dark matter."

The Dragonfly Telephoto Array is good at spotting dim objects that other telescopes might miss
The Dragonfly Telephoto Array is good at spotting dim objects that other telescopes might miss

The strange galaxy came to the team's attention when it was noticed that NGC1052–DF2 looked very different through Yale's Dragonfly Telephoto Array telescope, compared to how it appears in the Sloan Digital Sky Survey data. To study the galaxy further and calculate its mass, the team followed it up with spectroscopic observations using the Keck Observatory in Hawaii, as well as the Hubble Space Telescope.

While the discovery may sound like a blow to the theory of dark matter, the researchers say it instead strengthens the case for the mysterious substance. Since dark matter was inferred to plug holes between Newton's laws of motion and what was actually observed, some scientists have suggested that the laws themselves need revision. But, the Yale researchers argue, if that were the case, then that mass discrepancy should apply to every galaxy.

The find does raise further questions of course. It was thought that dark matter and ordinary matter generally clump together in the same places in the universe, since the latter is attracted to the gravitational pull of the former. How NGC1052–DF2 formed without dark matter is uncertain – as is the origin of the galaxy Dragonfly 44, which has the exact opposite problem of being composed of 99.99 percent dark matter.

"We thought that every galaxy had dark matter and that dark matter is how a galaxy begins," says van Dokkum. "So finding a galaxy without it is unexpected. It challenges the standard ideas of how we think galaxies work, and it shows that dark matter is real. It has its own separate existence apart from other components of galaxies. This result also suggests that there may be more than one way to form a galaxy."

Obviously we still have much to learn, but it's fascinating to think that one of the strongest pieces of evidence for dark matter to date is its own absence.

The research was published in the journal Nature.

Source: Yale University

11 comments
Mzungu_Mkubwa
No where near being an astrophysicist, so my comments here have less than no consequence, but I can't help but wonder that most of the discrepancies calculated are due to the immense distances that the light we observe here and now has traveled to get here, and what may or may not have happened to it during that journey. To say that the photons that ultimately arrive in our instruments remained pristine & untouched by *any* force during their billion(s)-plus-year journey across space seems incredibly naive to me. I suppose the fact that what we observe in the heavens doesn't look like old analog "noise" that used to show up on our CRT TVs between UHF/VHF channels tells us there is some coherency, but still... That's a lot of time and a lot of distance to cover, right?
mpc755
Dark matter fills 'empty' space, strongly interacts with visible matter and is displaced by visible matter. The reason for the 'missing dark matter' is that the galaxy is so diffuse that it doesn't displace the dark matter outward and away from the galaxy to the degree that the dark matter is able to push back and cause the stars far away from the galactic center to speed up. It's not that there is no dark matter connected to and neighboring the visible matter. It's that the galaxy is not well defined enough to displace the dark matter to such an extent that it forms a 'halo' around the galaxy. A galaxy's halo is not a clump of dark matter traveling with the galaxy. A galaxy's halo is the state of displacement of the dark matter.
Bob
So, is our sun's evil twin dark matter? Is the missing tenth planet in our solar system dark matter? Is dark matter really just cold ordinary matter? If they can't find it in our immediate area of the universe it would seem that several theories including the big bang need to be reconsidered.
apprenticeearthwiz
The exception that proves the rule. When that saying came about it meant the exception that tests the current rule, potentially changing it. 'Dark' is scientist speak for 'we have no information whatsoever'. That applies to approximately 80% of the universe. Huge respect to Stephen Hawking who was spot on when he said, "We are just an advanced breed of monkey on a minor planet of a very average star. But we can understand the universe and that makes us very special indeed." With one minor exception. We can contemplate the universe. Still incredibly special but we are far from understanding it.
Jerry May
I am not surprised since much guesswork as it turns out is askew, beginning with the notion the the universe suddenly materialized via the big bang or creation. Come on geniuses. If you explode something, the far flung fragments aren't older than the ones that fell nearby. They are the same age. Back to the calculator.
Gene
Martin Lo at JPL and I have been studying the need for an extra 1/r force term which when added eliminates the need to have the CDM. My notes on this are posted here http://egpreston.com/GravityMod.pdf describes our work. The odd galaxy seems to lack a black hole or has a very low mass one. Galaxies with super massive black holes are controlled by the black hole through the 1/r term and gives them their shape as well as the spiral arms. We are working on simulation models to prove this.
Douglas E Knapp
Jerry May not be right. The age of a thing has to do with how much time has passed for it. Time does not run at the same speed everywhere, it is related to the amount of mass at that spot. So you idea about an explosion and everything being the same age is wrong and shows a fundamental high school level lack of info about how time, mass and space work.
bwana4swahili
"the find may just turn out to be the exception that proves the rule" Based on proper English this would mean Dark Matter doesn't exist! Probably correct. I'm a believer in the KISS principle (of science). Dark Matter is an old idea and too complicated to be in the overall scheme of things. All one has to do is tweak gravity a touch and Dark Matter is no longer required!
Douglas Bennett Rogers
This bears the same relation to dark matter as supernova 1987a bears to neutrinos. In the case of the supernova, the neutrino pulse came at precisely the right time and in the right amount to be a neutrino pulse from the nuclear reaction required by the theory.
Si77
Could the missing mass be either orbiting neutron stars, small black holes or, less likely, multitudes of planets, or a combination of these non-emitting bodies?
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