In the ongoing quest to understand dark matter, collisions between clusters of galaxies provide a great testing ground. We learned quite a bit about dark matter from observations of the Bullet Cluster, and now a new study provides further insight by looking at dozens of galaxy cluster collisions, further constraining models of dark matter in which the individual particles collide with each other.

The conventional model of dark matter is that it’s cold—meaning that the particles are moving relatively slowly—and the particles don’t interact with each other much, if at all. This model has for the most part been incredibly successful in predicting a wide range of phenomena, from the behavior of galaxies up to the large-scale structure of the Universe. There is, however, some data which seems to conflict with the model. For one thing, the model predicts that there should be higher dark matter density toward the centers of galaxies and galaxy clusters, and our observations do not match those predictions.

There are a number of possible resolutions to this conflict. Some of them involve potential processes involving the ordinary (baryonic) matter in those galaxies. Others involve slightly different versions of the cold dark matter model. One possibility in the latter category is that dark matter is self-interacting—dark matter particles can collide with each other more often than is commonly thought.

Self-interacting dark matter While we don’t yet know which particle makes up dark matter, one characteristic we do know is that it doesn’t interact via the electromagnetic force. Electromagnetism is the force that holds atoms together, the one which prevents you from falling through the floor as you read this. Your constituent particles are far enough apart that, if not repelled by this force, they’d slip right between the floor’s particles. So if a particle such as dark matter were unaffected by electromagnetism, its particles could (and do!) pass right through the Earth as though nothing was there. While we don’t yet know which particle makes up dark matter, one characteristic we do know is that it doesn’t interact via the electromagnetic force. Electromagnetism is the force that holds atoms together, the one which prevents you from falling through the floor as you read this. Your constituent particles are far enough apart that, if not repelled by this force, they’d slip right between the floor’s particles. So if a particle such as dark matter were unaffected by electromagnetism, its particles could (and do!) pass right through the Earth as though nothing was there. Furthermore, the lack of interaction via the electromagnetic force is the reason for the "dark" part of "dark matter"—light is really just electromagnetic waves, so it doesn’t interact with them. If dark matter particles don’t interact electromagnetically, it’s less likely for them to interact with one another—but not necessarily impossible. It’s also possible that dark matter interacts via the weak force, another of the four fundamental forces. The reach of the weak force is very short, but it helps control the particle interactions that build or tear apart atomic nuclei. If dark matter turns out to be self-interacting, it would be a good thing for astronomers. For one thing, these interactions could produce gamma rays which would be detectable. We recently reported on an observation of a gamma ray signal from a dwarf galaxy, which could potentially be the result of dark matter interactions. Self-interacting dark matter could also go a long way toward solving a few remaining problems with the cold dark matter model.

Cluster collisions

Colliding galaxy clusters make an excellent testing ground for our ideas about dark matter. The majority of each cluster’s mass is dark matter, so where the visible matter collides, the dark matter is present as well. And when two massive clumps of dark matter move together, the resulting interactions can teach us a lot about how that dark matter behaves and whether (and how much) dark matter is self-interacting. So an international research team set out to build a collection of observations of galaxy cluster collisions; they describe their results in a paper in this week's Science.

“If dark matter’s particle interactions are frequent but exchange little momentum,” the authors explain in their paper, “the dark matter will be decelerated by an additional drag force. If the interactions are rare but exchange a lot of momentum... dark matter will tend to be scattered away.”

Based on these criteria, it should be possible to find the dark matter cross-section, which is the frequency of the dark matter particle interactions. All that’s necessary is to measure the motions of the dark matter and determine whether it’s been decelerated or scattered by frequent self-interactions.

Previous studies have used this technique to put constraints on the cross-section. The tightest constraints so far come from observations of the Bullet Cluster—one of the best-known dark matter observations. By measuring the drag on the dark matter in that cluster, researchers were able to narrow down the maximum possible cross-section to 1.25 square centimeters per gram. In order to solve the residual issues with the cold dark matter model, the cross-section would have to be somewhere between 0.1 and one square centimeter per gram.

Since the Bullet Cluster, studies of other galaxy cluster collisions have attempted to further constrain this cross-section—but they’ve failed. Until now, the Bullet Cluster has remained the tightest constraint on dark matter’s behavior.

This is because the analysis of any single system has an inherent difficulty: it’s very challenging to model the three-dimensional geometry of the object being observed. Galaxy clusters that are colliding with each other are complicated messes, and the angle of the clusters’ motion with respect to our line of sight, as well as their impact velocities, aren’t easy to determine.

A statistical method

The new study has found a way to work around these problems. While individual measurements are messy, the authors have used a statistical method that evaluates many galaxy cluster collisions at once, thus canceling out some of the messiness. Digging into the data archives of the Hubble Space Telescope and the Chandra X-Ray Observatory, the authors examined every galaxy cluster system they could find, then chose 30 galaxy cluster systems that met specific criteria.

Combining the measurements from all those systems, the researchers were able to determine the average amount of lag on the clusters’ dark matter relative to the motion of the clusters’ gas. With that information in hand, the researchers further constrained the dark matter cross-section; it’s less than 0.47 square centimeters per gram.

This constraint marks a big improvement over the previous value of 1.25 from the Bullet Cluster, and it rules out certain candidates for dark matter particles beyond the Standard Model. Among these ruled-out candidates are so-called mirror matter and a dark force affecting dark matter.

These findings are significant, because the search for the dark matter particle—if it is indeed a particle—is an ongoing struggle which plays into our understanding of the Universe at large. Currently, WIMPs are the leading candidate, but there are many other possibilities. Narrowing that list is an important step forward.

The researchers were also careful not to assume that dark matter exists in the first place, so before extrapolating the cross-section, they checked to make sure the observed motions couldn’t have occurred without the dark matter. They concluded, with a strong 7.6 sigma confidence, that dark matter does indeed exist in the clusters observed. If there was still in doubt in anyone’s mind, this should help alleviate it—the case for dark matter just got even stronger. (And it was already pretty darn strong).

The method they’ve used can also be reused as more data becomes available. “This inherently statistical technique can be readily expanded to incorporate much larger samples from future all-sky surveys,” they write in their paper.

Edit: The article originally implied that the new observations provide new evidence for self-interacting dark matter, when in fact it further constrains it. The first paragraph was changed to reflect that.

Science, 2015. DOI: DOI: 10.1126/science.1261381 (About DOIs)