For decades, it has been the mysterious, invisible material that makes up most of the cosmos. Now, astronomers have produced the strongest evidence that “dark matter” really exists, after initially writing off the discovery as an error in their measurements.
A team using Nasa’s Hubble Space Telescope has discovered a ghostly ring of dark matter that formed long ago during a titanic collision between two clusters of galaxies.
It created a remarkably uncluttered picture of an elusive substance that was first postulated in 1933 and provides new opportunities to figure out what it is.
Astronomers have long suspected the existence of the invisible substance as the source of additional gravity from “dark matter”.
Calculations suggest such clusters would fly apart if they relied only on the gravity of their visible stars.
There was tremendous excitement in January at the American Astronomical Society in Seattle when it was shown that dark matter forms an invisible “skeleton” around which the “flesh” of the ordinary matter of stars and galaxies is hung.
But the problem has been that dark matter sits alongside ordinary matter in galaxies, along with the thin veil of dust between cosmic objects, making it hard to reveal.
The dark ring has now been published in the Astrophysical Journal. “This is the first time we have detected dark matter as having a unique structure that is different from both the gas and galaxies in the cluster,” said astronomer Dr James Jee of Johns Hopkins University, Baltimore, a member of the team that spotted the dark matter ring.
“By seeing a dark matter structure that is not traced by galaxies and hot gas, we can study how it behaves differently from normal matter.”
The researchers unexpectedly stumbled across the ring while they were mapping the distribution of dark matter within the galaxy cluster Cl 0024+17 (ZwCl 0024+1652), located five billion light-years from Earth. The ring measures 2.6 million light-years across.
Tracing dark matter is not an easy task, because it does not shine or reflect light. Astronomers can only detect its influence by how its gravity affects light.
To find it, astronomers study how faint light from more distant galaxies is warped and smeared into arcs and streaks by the gravity of the dark matter in a foreground galaxy cluster. This powerful phenomenon is called gravitational lensing and was first predicted by Albert Einstein.
By mapping the distorted light, astronomers can deduce the cluster’s mass and trace how dark matter is distributed in the cluster.
During the team’s dark-matter analysis, they noticed a ripple in the mysterious substance, somewhat like the ripples created in a pond from a stone plopping into water.
“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,” Dr 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.”
Curious about why the ring was in the cluster and how it had formed, Dr Jee found previous research that suggested the cluster had collided with another cluster between one and two billion years ago. Astronomers have a head-on view of the collision because it occurred fortuitously along Earth’s line of sight.
From this perspective, the dark-matter structure looks like a ring.
“The collision between the two galaxy clusters created a ripple of dark matter that left distinct footprints in the shapes of the background galaxies,” Dr Jee explained.
Computer simulations of galaxy cluster collisions, created by the team, show that when two clusters smash together, the dark matter falls to the heart of the combined cluster and sloshes back out. As the dark matter moves outward, it begins to slow down under the pull of gravity and pile up, like cars bunched up on a motorway.
“By studying this collision, we are seeing how dark matter responds to gravity,” said team member Prof Holland Ford, also of Johns Hopkins University.
“Nature is doing an experiment for us that we can’t do in a lab, and it agrees with our theoretical models.”
Mapping dark matter’s distribution in space and time is fundamental to understanding how galaxies grew and clustered over billions of years.
Tracing the growth of clustering in dark matter may eventually also shed light on dark energy, a repulsive form of gravity that would have influenced how dark matter clumps.
Many scientists now feel that we are on the verge of discovering what it is that has formed such an immense part of the universe.
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