Dark matter has to be made up of some sort of elementary particle, but we know astoundingly little about it. Its existence can be inferred from its necessary effects — something we can ‘t see seems to be holding galaxy clusters together, because the gravity from the stars we do observe in them isn’t sufficient to do the job. That makes gathering any evidence for dark matter’s behavior — indeed, for its very existence — a crucial goal for astrophysicists. And today we have the strongest supporting evidence yet that dark matter is real.
The work comes via the Hubble Space Telescope, used by a team of astronomers to locate what appears to be a ring of dark matter in the cluster ZwCl0024+1652, some five billion light years from our Solar system. The ring is 2.6 million light years across, and this detection appears to be unique. Says M. James Jee (Johns Hopkins): “This is the first time we have detected dark matter as having a unique structure that is different from the gas and galaxies in the cluster.” Lee was a member of the team that found the dark matter ring.
Image: The galaxy cluster Cl 0024+17 (ZwCl0024+1652) as seen by Hubble’s Advanced Camera for Surveys. The image displays faint faraway background galaxies that had their light bent by the cluster’s strong gravitational field. By mapping the distorted light and using it to deduce how dark matter is distributed in the cluster, astronomers spotted the ring of dark matter. One of the background galaxies is located about two times further away than the yellow cluster galaxies in the foreground, and has been multiple-imaged into five separate arc-shaped components, seen in blue. Credit: NASA, ESA, M.J. Jee and H. Ford (Johns Hopkins University)
A possible cause for the ring: The collision between two galaxy clusters some 1 to 2 billion years ago. Using earlier evidence of such a collision, the team’s computer simulations modeled the event, showing how any associated dark matter might react. In a way, says Holland Ford (also of Johns Hopkins), “Nature is doing an experiment for us that we can’t do in a lab, and it agrees with our theoretical models.”
Let’s be clear on the method here. Dark matter, by its nature, cannot be seen. Astronomers infer its existence by observing how its gravity bends the light of distant background galaxies. We’ve looked at that phenomenon, called gravitational lensing, before. It’s a powerful tool not only in the study of cosmological structure but is also helpful in exoplanet detections. Such lensing is a major way to learn about dark matter, but in this case the substance — whatever it is — is widely separated from the gas and galaxies that make up the clusters themselves.
That gives astronomers a chance to home in on qualities particular to dark matter, the things that distinguish it from the ordinary matter — stars, planets, people — that make up the 4 percent of the universe we can actually see. And what has shown up is an unexpected ‘rippling’ effect that caused ripples of its own among team members:
“I was annoyed when I saw the ring because I thought it was an artifact, which would have implied a flaw in our data reduction,” Jee explained. “I couldn’t believe my result. But the more I tried to remove the ring, the more it showed up. It took more than a year to convince myself that the ring was real. I’ve looked at a number of clusters and I haven’t seen anything like this.”
Jee goes on to explain the ripple effect:
“The collision between the two galaxy clusters created a ripple of dark matter which left distinct footprints in the shapes of the background galaxies. It’s like looking at the pebbles on the bottom of a pond with ripples on the surface. The pebbles’ shapes appear to change as the ripples pass over them. So, too, the background galaxies behind the ring show coherent changes in their shapes due to the presence of the dense ring.”
So now we’re seeing dark matter in a new kind of distribution, offering further clues about its nature. I sometimes think of dark matter (and the equally mysterious dark energy) as needed correctives to the notion that we are within a few years (or decades, perhaps) of truly understanding the cosmos. That point of view, which settles in every few centuries only to be disrupted by new scientific breakthroughs, now needs the adjustment supplied by a simple fact: We have no good explanation for more than a fraction of the matter that pervades the universe, nor do we fathom the relentless acceleration now thought to fuel its expansion.
An excellent video on these findings is available here. Younger readers who are contemplating a career in astrophysics should rush to get into work like this. We are entering an era of unprecedented discovery.