The Bullet galaxy is a smoking gun for dark matter (blue) (Image: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI)

The smallest galactic smash-up to show evidence of dark matter has been found, and it hints at an army of similarly tiny lookalikes.

Dark matter is the mysterious stuff that is thought to make up about 83 per cent of the universe’s matter. So far, we haven’t seen it interact with ordinary matter except through gravity. But we suspect it is there because galaxies seem to have more mass than we can account for with visible matter alone.

One of the most famous smoking guns for dark matter is the Bullet cluster (see image above). Here, a small galaxy cluster swept through a larger one about 100 million years ago. The galaxies passed far enough apart not to hit, but the hot gas (pink) in between them collided and pooled on the trailing ends of each cluster. Maps of mass distribution show there must be dark matter (blue) as well as visible matter, and that it travelled faster than the gas, separating out to keep up with the galaxies.


Similar examples have since been found but only in a handful of very massive clusters. “It was thought that this kind of separation could happen only when there were massive clusters – big guys smashing together,” says Fabio Gastaldello at the National Institute for Astrophysics in Milan, Italy. Now Gastaldello and his team have found much smaller groups in the process of merging that also show separating dark matter.

Hail of bullets

The team found the colliding galaxy groups in survey images from the Canada-France-Hawaii Telescope on Mauna Kea in Hawaii. The relatively small cluster distorted the images of other galaxies behind it. Astronomers see this effect when the gravity of massive objects distorts and magnifies light in the background, creating what’s called a gravitational lens. Seeing this cluster act as a lens showed that it contains a lot of unseen mass, probably tied up in dark matter.

Follow-up images showed where the galaxies are located and traced the locations of intergalactic gas and dust. As in the Bullet cluster, dark matter is lumped with the visible galaxies and hot gas is trailing behind. But the total system is seven times less massive than the Bullet cluster.

Small galaxy groups like this one are about 1000 times more common than large clusters, so there should be many more waiting to be discovered in upcoming surveys.

“The famous Bullet cluster is the merging of two massive things moving very fast, so it’s very rare,” says James Bullock at the University of California, Irvine. “The fact that they were able to measure a dark matter offset in a smaller cluster is important because it means we might be able to build statistics.”

Any individual cluster or group can give a sense of how strongly dark matter interacts with ordinary matter or with itself. But with only a few systems in hand, scientists can’t tell if the results they get apply broadly or just to that cluster. “If we had many, many more of them, then you can beat down those uncertainties and build reliable constraints,” says Bullock.

Reference: arxiv.org/abs/1404.5633