It’s hot. It’s crowded. And it’s one of the most raucuous space parties astronomers have ever seen.
A research team using a combination of three powerful telescopes is spilling the beans on the galaxy cluster MACSJ0717.5+3745 (MACSJ0717 for short), located about 5.4 billion light years from Earth. The wild system contains four separate galaxy clusters undergoing a triple merger — the first time such a phenomenon has been documented — and that’s just the beginning.
Galaxy clusters are the largest objects bound by gravity in the Universe. Using data from NASA’s Chandra X-ray Observatory, Hubble Space Telescope and the Keck Observatory on Mauna Kea, Hawaii, astronomers were able to determine the three-dimensional geometry and motion in MACSJ0717.
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Its 13-million-light-year-long stream of galaxies, gas and dark matter — known as a filament — is pouring into a region already full of galaxies. Like a freeway of cars emptying into a full parking lot, this flow of galaxies has caused one collision after another.
“In addition to this enormous pileup, MACSJ0717 is also remarkable because of its temperature,” said lead author Cheng-Jiun Ma, of the University of Hawaii. “Since each of these collisions releases energy in the form of heat, MACS0717 has one of the highest temperatures ever seen in such a system.”
While the filament leading into MACJ0717 had been previously discovered, these results show for the first time that it was the source of this galactic pummeling. The evidence is two-fold. First, by comparing the position of the gas and clusters of galaxies, the researchers tracked the direction of clusters’ motions, which matched the orientation of the filament in most cases. Secondly, the largest hot region in MACSJ0717 is where the filament intersects the cluster, suggesting ongoing impacts.
“MACSJ0717 shows how giant galaxy clusters interact with their environment on scales of many millions of light years,” said team member Harald Ebeling, also from the University of Hawaii. “This is a wonderful system for studying how clusters grow as material falls into them along filaments.”
Computer simulations show that the most massive galaxy clusters should grow in regions where large-scale filaments of intergalactic gas, galaxies, and dark matter intersect, and material falls inward along the filaments.
“It’s exciting that the data we get from MACSJ0717 appear to beautifully match the scenario depicted in the simulations,” said Ma.
In the future, Ma and his team hope to use even deeper X-ray data to measure the temperature of gas over the full 13-million-light-year extent of the filament. Much remains to be learned about the properties of hot gas in filaments and whether infall along these structures can significantly heat the gas in clusters over large scales.
“This is the most spectacular and most disturbed cluster I have ever seen,” says Ma, “and we think that we can learn a whole lot more from it about how structure in our Universe grows and evolves.”
The paper describing these results appeared in the March 10 issue of Astrophysical Journal Letters.