What's Making This Blob of Dark Matter Slow Down?

​Recent observations made by the Hubble Space Telescope and the European Southern Agency's Very Large Telescope (VLT) have revealed something unexpected: the mass distributions within and around four distant colliding galaxies are not as they should be. Or, rather, they're not as they should be given a common assumption about the nature of dark matter—namely, that dark matter does not interact with itself.

These results come courtesy of Durham University computational cosmologist and lead investigator Richard Massey and an international team. Their work, which is published in the current Monthly Notices of the Royal Astronomical Society—while available gratis from the ESO—offers the possibility of "rich physics in the dark realm" and a significant upending of some cosmological assumptions.

The darkness of dark matter has everything to do with interaction. And, within physics, there are four fundamental interactions, with the most everyday of those (to us, as large collections of atoms and biology) being electromagnetism. This is where photons and electrons are traded around, forming the basis of light itself. And most of the internal properties and macro-scale forces we experience grow from this foundation: light, electricity, magnets.

Dark matter is often thought to be electromagnetically agnostic. Cosmologists and astrophysicists are most often in favor of what are known as weakly interacting massive particles (WIMPS), which are a lot like they sound. Dark matter might indeed be 85 percent of all matter in the universe—which it needs to be to sufficiently explain the observed universe—but without electromagnetism, it's limited to existence as a cosmological ghost, influencing the universe through the very large scale and blunt force of gravity, but not much else.

WIMPs are gradually falling out of favor, mostly because we haven't found them in the places they should be. This makes it an especially good time for theories like interacting dark matter, which is where dark matter is capable of some slight interactions, even with itself. The new results don't say very much about what variety of dark matter we're actually dealing with, specifically, but they do suggest some degree of self-interaction.

This is revealed through the relative distributions of visible light and inferred mass among distant galaxies, particularly as those distant galaxies interface with each other through collisions. Dark matter is after all the gravitational foundation or lattice upon which galaxies are built. Galaxies are nested within a halo of dark matter and it's this halo that keeps galactic material from spinning off into space. Our solar system and, eventually, humans exist because of dark matter.

So when galaxies collide, dark matter collides as well. If dark matter did not interact with itself, then one galaxy's halo would just slip through the other galaxy's halo unimpeded. However, Massey and his team—building on earlier low resolution, ground-based observations of the same cluster, Abell 3827—found that this is not the case. In at least one of the four studied galaxies, dark matter seems to be lagging behind, suggesting some resistance and, thus, interaction. Why can't this one batch of dark matter keep up with its galactic mate?

When a galaxy collides with another galaxy or galactic cluster, the effect is that eventually all of its gas content get smoothed out as the result of increasing pressure, like a windblown sand dune. "Full numerical simulations predict that the dark matter is eventually smoothed, but disagree about the timescale and the radius/orbits on which stripping occurs," Massey and company note in the current study.

Abell 3827 turns out to be the perfect place to study this, in part because it has a "unique strong gravitational lens system" that allows its dark matter to be mapped according to its readily observable gravitational effects on passing light, e.g. through gravitational lensing. The purplish smudges in the image above are actually light from some semi-obscured galaxy as it bends in response to a gravitational source.

Massey and his team think that the dark matter halo in question might be lagging behind because it's experiencing some unexpected friction, possibly from the halos of the other galaxies involved in the collision. It's a nice explanation but still not definitive. As the researchers note, the offset between the galaxy and its halo is small enough such that some other so-far unseen interaction can't be ruled out. And even if it is self-interacting dark matter, the nature of that interaction is anyone's guess. It could be something well-known or it could be something entirely exotic. The one thing that's for sure is that the interaction is not caused by gravity.

Finally, Massey points out one crucial uncertainty in his research. The dark matter lag could be the result of a very, very small effect occurring over a very large period of time, or it could be a large effect occurring at the relatively short time scales of hundreds of millions of years.

If all of this sounds a bit familiar, it's because just a few weeks ago a group of astronomers published a paper in Science finding that among a much larger set of galactic clusters (72), there seemed to be basically no dark matter-dark matter self-interaction at all, at least with regard to exchange of momentum. This doesn't completely disallow the new results, but it would seem to constrain their interpretation significantly, suggesting that the frictional force observed comes from something less traditional than regular old collisions between particles.

More specifically, any dark matter self-interaction would have to come at levels below that of proton-proton interactions, which doesn't eliminate interaction entirely. The resolution remains to be seen.