This new image of super-cluster Abell 2744 captures the wreckage of a collision between four smaller galaxy clusters. New data let astronomers map the positions of three different kinds of matter in the system, which may offer clues to how dark matter behaves when it smacks into ordinary matter.
The cluster, nicknamed "Pandora's Cluster" for its bizarre complexity, is as massive as 1.8 quadrillion suns and contains trillions of stars. Only about 5 percent of that mass is made up of galaxies that glow in visible light.
The rest of the mass is gas (20 percent, shown here in pink) that's so hot it's only observable through the X-rays it emits, and dark matter (75 percent, blue), which is utterly invisible.
To figure out dark matter's location, astronomers looked for the telltale stretching of galaxies located far behind the cluster. Huge masses warp the shape of space-time in their vicinity and bend the path of light, a phenomenon called gravitational lensing. By carefully plotting how galaxies in the background are distorted, astronomers can map where the invisible mass of dark matter lies.
To make this map, the most detailed to date, astronomers used new data from the Hubble Space Telescope and the Very Large Telescope in Chile. They used pre-existing data from NASA's Chandra X-ray Observatory to locate the hot gas in the cluster.
"We find that it is even more complex than previously thought, unleashing a variety of exciting effects," the researchers wrote in a paper to be published in the Monthly Notices of the Royal Astronomical Society (PDF).
The researchers identified four distinct sub-clusters, each with a mass of about 100 trillion suns. They suggest that the cluster is the result of two near-simultaneous mergers between four separate galaxy clusters around 350 million years ago. The smash-up sent galaxies, dark matter and gas flying in all directions. Some sections were left with lots of galaxies and dark matter but no gas, others with lots of dark matter but no regular matter.
"The separation between all three mass components makes this a real puzzle," the researchers write. "It should be stressed that such a peculiar configuration is observed for the first time."
Near the center of the image lies a bullet-shaped clump where the gas of one cluster collided with that of another to create a shock wave -- but dark matter passed right through the collision unaffected. This separation of dark matter and ordinary matter was previously observed in the Bullet Cluster, but the two types of matter are much further separated in Pandora's Cluster.
Another weird feature is the lone chunk of gas toward the upper right of the image, which the researchers call the "ghost" cluster. Typically, astronomers expect dark matter to take the lead after a collision, and gas to follow after it. But the ghost cluster somehow got ahead of its associated dark matter.
The researchers think the gas could have been flung outward as the dark matter slowed down and cooled after the collision. "However, this scenario needs further confirmation," they write.
More detailed studies of Pandora's puzzling configuration could help pin down what exactly dark matter is, and how it helped shape the evolution of the universe.
Image: NASA, ESA, ESO, CXC & D. Coe (STScI)/J. Merten (Heidelberg/Bologna)
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