There鈥檚 a strange galaxy in the distant universe and it鈥檚 missing something big: dark matter. At the edges of most galaxies, stars orbit so quickly that they should be flung away. The fact that they don鈥檛 implies that these galaxies have far more mass than we can see, and the gravity of that extra mass 鈥� dark matter 鈥� holds them together.
Some聽galaxies are up to 99.8 per cent dark matter, says at Yale University. But he and his colleagues found one that has at least 400 times less of this mysterious substance.
We can tell a galaxy鈥檚 total mass by measuring the velocities of stars or other objects in a galaxy. Subtracting the mass of the visible matter from that total will reveal the amount of dark matter there. When van Dokkum and his team went through this process for a galaxy they discovered about 65 million light years away 鈥� called NGC1052-DF2, or just DF2 鈥� they found that it聽likely has聽no dark matter at all.
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鈥淭here have been a few claims of galaxies without dark matter over the last few decades, but they鈥檝e all kind of gone away with more careful measurements,鈥� says van Dokkum. In most of those galaxies, researchers measured the mass of the central, star-dominated region, but dark matter was hiding towards the edges. 鈥淚n this particular galaxy, because it鈥檚 so big and diffuse, there鈥檚 nowhere for the dark matter to hide,鈥� he says.
Stars in motion
Van Dokkum and his colleagues used the motions of 10 clusters of stars around the galaxy鈥檚 centre to find a total mass聽of approximately 340 million times the mass of the sun. That is an upper limit, so there is a 90 per cent chance that it鈥檚 actually smaller. The mass of the stars in DF2 is about 200 million solar masses, plus or minus 100 million solar masses.
These numbers are imprecise because the galaxy is so distant and dim, but if the galaxy is less massive than the upper limit and the visible mass is at the highest possible level, there鈥檚 no room left for dark matter. And von Dokkum鈥檚 simulations show this is the most likely scenario. If there is any at all, it is less dark matter than expected by a factor of about 400. Either way, the consequences of their discovery remain the same.
It may do away with some theories of modified gravity which posit that there is no dark matter. These ideas suggest that galaxies don鈥檛 fly apart because gravity behaves differently in the outer reaches of galaxies than it does in our part of the universe, which would account for the weird stellar motions that seem to indicate dark matter.
But a galaxy without any signs of dark matter, like this one, also has no signs of modified gravity. 鈥淚t鈥檚 counterintuitive, but the absence of dark matter here is actually proof of its existence elsewhere 鈥� it鈥檚 a real substance that can be associated with a galaxy, or not,鈥� says van Dokkum. That is, dark matter isn鈥檛 a force that is applied evenly across the cosmos, but tangible matter that can clump in various ways.
Birth of a galaxy
The lack of dark matter in DF2 may also upend our ideas of how galaxies form. 鈥淕alaxies begin as a blob of dark matter that accretes gas, which turns into stars, which turn into galaxies,鈥� says van Dokkum. So how did this one come to be?
The researchers say it could be made of a cloud of gas flung out of a collision between two other galaxies, or a pool of gas could have been contained by high-speed winds blowing off black holes in the early universe. But these events are extremely rare, so if DF2 isn鈥檛 the only galaxy of its kind, they might explain how it formed.
鈥淚f we find that such a class of galaxies is fairly common, we鈥檙e going to have to come up with a consistent explanation of how they form,鈥� says at the Institute of Advanced Study in Princeton, New Jersey.
Or we may need revise our theories of how dark matter infuses the cosmos, says at Princeton University. 鈥淭his either indicates some new and unusual way of forming galaxies, or it鈥檚 a clue that our standard picture of how dark matter works is wrong,鈥� he says.
Nature
Read more: Ghost galaxy is 99.99 per cent dark matter with almost no stars
