THERE is a strange, distant galaxy that seems to be missing something big: dark matter.
Stars at the edges of most galaxies orbit so quickly that they should be flung away. They aren鈥檛, though, which implies that the gravity of some unseen extra mass 鈥� thought to be dark matter 鈥� holds these galaxies together.
We measure a galaxy鈥檚 total mass using the velocities of the stars it contains. Subtracting the mass indicated by the amount of the stars鈥� light from that total reveals how much dark matter the galaxy might have. When at Yale University and his colleagues did this for a galaxy called NGC 1052-DF2 that is some 65 million light years away, they found that it probably has no dark matter at all.
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In some galaxies, dark matter hides towards the edges. But 鈥渋n this particular galaxy, because it鈥檚 so big and diffuse, there鈥檚 nowhere for the dark matter to hide鈥�, van Dokkum says. The team found that DF2 is about 340 million times the mass of the sun. That is an upper limit, so there is a 90 per cent chance that it is smaller. The mass of the stars in DF2 is between about 100 million and 300 million solar masses (Nature, ).
The numbers are imprecise because the galaxy is so distant and dim, but if its mass is below its upper limit and the visible mass is at the top end of its range, there is no room left for dark matter. The team鈥檚 simulations indicate that this scenario is likely. If there is any dark matter there, it is just 1/400th of what is expected in a such a diffuse galaxy.
Either way, the discovery has the same consequences: it may kill off some theories of modified gravity. These posit that there is no dark matter and galaxies hang together instead because gravity acts differently in their outer reaches from how it works in our part of the universe. That would account for the quickly orbiting stars that seem to indicate dark matter exists.
鈥淚t鈥檚 counter-intuitive, 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.
A lack of dark matter in DF2 may also change our ideas of how some 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. If that鈥檚 the case, how did DF2 come to be? Van Dokkum says it could have formed as a cloud of gas was 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.
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� says at Princeton University.
This article appeared in print under the headline 鈥淎 galaxy missing its dark matter鈥�