Thanks to a hard collision between galaxies, the existence of dark matter is more likely again

Astronomers have found an explanation for two galaxies that contain hardly any dark matter, a riddle that has puzzled astronomers for years. With a homemade telescope, Dutch astronomer Pieter van Dokkum found two galaxies in 2018 and 2019 that had almost no dark matter in them. Now, model simulations by the same team show that dark matter is missing there, because older galaxies from which they formed collided eight billion years ago. Dark matter separated from gas, and new galaxies without dark matter arose from that gas. The team published the results Wednesday in the magazine Nature†

Dark matter has never been seen, smelled or heard. Its existence follows from calculations of rotational speeds in the cosmos. Galaxies spin so fast that astronomers expect them to swing apart. There isn’t enough mass in a galaxy to generate the gravity needed to hold it together. That’s why hypothetical stuff has been created, dark matter, that generates the necessary gravity. The common idea was that galaxies could only form with dark matter.

But galaxies DF2, found by the team of Van Dokkum, professor at the American Yale University, contains hardly any dark matter. The galaxy is spinning so slowly that it doesn’t need dark matter to hold it together. DF2 is about 65 million light-years away and orbits the larger galaxy NGC 1052. Van Dokkum found the galaxy with the Dragonfly, a telescope with 48 lenses that he built together with a Canadian colleague.

A year later, Van Dokkum’s team found another galaxy without dark matter. That was DF4, and it also orbits NGC 1052.

Gas-rich galaxies

Contrary to popular belief, how is it possible that galaxies form without dark matter? In 2019, astronomer Joseph Silk suggested that such a galaxy is the result of two colliding, gas-rich galaxies. In the young cosmos, galaxies consisted mainly of dark matter and gas. When such systems collide, everything in them is shaken up. Dark matter and any stars move past each other and eventually drift out. But gas is compressed and therefore floats out much more slowly. Thus, gas separates from dark matter. New stars are formed from the gas and eventually a new galaxy, without dark matter.

Van Dokkum’s model simulations now confirm that DF2 and DF4 may have arisen in this way. Van Dokkum: „We investigated the current movements and speeds with which DF2 and DF4 move away from each other and rewinded them in time. Those movements correspond to simulated movements of the galaxies that were formed from a collision between other galaxies, about eight billion years ago.”

It must have been a violent collision. Only when galaxies collide at least 300 kilometers per second will the collision be hard enough to separate dark matter from gas. A high impact speed is consistent with the observations. DF2 and DF4 are still moving away from each other at a rapid pace, 358 kilometers per second.

“Surprisingly, we also found a row of six other galaxies in the same line as DF2 and DF4, probably also dark matter-less,” says the astronomer.

Strangely enough, dark matter-less galaxies actually provide evidence for the existence of dark matter. Van Dokkum: “Some scientists suggest that dark matter does not exist. They do not think that extra mass is needed to explain the amount of gravity, but argue that gravity itself works differently than we think. But if the laws of gravity were indeed different, you would expect this to be true for every galaxy. You can’t leave out the laws of physics in one place and not in another.”