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Galaxy cluster Abell 2744: The galaxies only provide around 5 percent of the cluster's mass. Hot intracluster gas (pink) is visible through its X-ray emissions. Otherwise invisible dark matter (around 75 percent of the mass) is indicated with the blue overlay. (NASA, ESA, ESO, CXC, D. Coe/STScI), J. Merten/Heidelberg-Bologna)
A cluster of distant colliding galaxies with some unusual characteristics has been spotted by an international team of astronomers and nicknamed "Pandora’s Cluster."
Otherwise known as Abell 2744, the cluster comprises galaxies believed to have collided with each other over a span of 350 million years.
"Like a crash investigator piecing together the cause of an accident, we can use observations of these cosmic pile-ups to reconstruct events that happened over a period of hundreds of millions of years," says lead author Julian Merten, in a press release.
"This can reveal how structures form in the Universe, and how different types of matter interact with each other when they are smashed together."
"We nicknamed it Pandora’s Cluster because so many different and strange phenomena were unleashed by the collision," says Renato Dupke, another scientist on the team, in the release. "Some of these phenomena had never been seen before."
To study the cluster, the team combined data from the NASA/ESA Hubble Space Telescope, the European Southern Observatory’s Very Large Telescope (VLT), the Japanese Subaru telescope, and NASA’s Chandra X-Ray Observatory.
Images from the optical telescopes revealed that galaxies comprise less than five percent of the cluster’s mass.
Most of the mass (75 percent) is actually dark matter which does not emit, absorb, or reflect light, but does exert gravitational forces that distort light passing through from distant galaxies. Mapping the distorted galaxies via gravitational lensing therefore pinpointed the dark matter.
The remaining mass consists of gases so hot they are only observable from the X-rays they release. Data from the Chandra X-ray Observatory revealed not only the distribution of the gases, but the directions and speeds at which the cluster components collided.
Unusually, there seem to be three very different zones around the core of Abell 2744, which the experts believe reveal clues about how dark matter and other forms of matter interact.
One zone is a "bullet" where the researchers believe gases collided and created a shock wave but where dark matter was unaffected.
In another part of the cluster, the hot gas seems to have somehow been stripped away, with only galaxies and dark matter visible.
In a third region, there seems to be a lot of dark matter, with a separate "ghostly" mass of hot gas near it.
There is still much to learn about the cluster. "Abell 2744 is undoubtedly undergoing a complicated merging process on a large cluster scale," according to the paper.
"Progressively more detailed studies, culminating in our lensing reconstruction and X-ray analysis, have only agreed that the merger is more complex than previously thought."The findings will be published in an upcoming issue of Monthly Notices of the Royal Astronomical Society.
