January 10, 2016
Using Keck, Hubble and Chandra telescopes, astronomers have discovered the most massive galaxy cluster in the early Universe - IDCS 1426. Although IDCS 1426 galaxy cluster, with its 500 trillion solar masses, is about 6 times less massive than the most massive galaxy cluster known, El Gordo, it is discovered at the distance of 10 billion light-years, meaning we see it as it was when our Universe was still very young.
Galaxy clusters are groups of galaxies held together by their mutual gravitational fields, created by both visible and dark matter they are made of as well as dark matter between them. Some of these galaxy clusters are so big, they are called galaxy superclusters.
The galaxy cluster called IDCS J1426.5+3508 (or just IDCS 1426) is 500 trillion times heavier than Sun and it lies about 10 billion light-years away. It was first discovered by the Spitzer Space Telescope in 2012.
Using data from Keck, Hubble and Chandra X-ray telescopes, team of astronomers led by Mark Brodwin of the University of Missouri, Kansas City, USA, has now managed to measure its distance and mass. Although we know of bigger galaxy clusters, like El Gordo with its 3 quadrillion solar masses, they are closer to us and thus older, meaning they had time to grow.
"Of all the structures we've ever seen, this is the most massive in the first 4 billion years of the Universe," says Brodwin. "It should be consistent with the largest cluster in the observable Universe."
Astronomers expect that as IDCS 1426 reaches El Gordo’s age, it will be just as massive, if not more. The previous largest known young galaxy cluster in the early Universe is Gioiello with 400 trillion solar masses, 9.6 billion light-years away.
Courtesy of Chandra X-ray telescope, Brodwin’s team found that 90% of this galaxy cluster's mass comes from dark matter. That is, however, not the most interesting thing about IDCS 1426 discovered with X-rays.
Temperature distribution inside IDCS 1426 shows that the hottest area is not the cluster’s core, but the region next to it. Normally, the core of a galaxy cluster is the most active region during its formation, because of objects bumping into each other and hot gas compressed into a relatively small area. As things settle down, core starts to cool off and emit X-rays.
The fact that IDCS 1426 has a cool core points to an evolved galaxy cluster, despite its age.
"A cool core is a property of an evolved cluster," Brodwin added.
It also points to a merger with another cluster some 500 million years earlier, which would displace the hot gas but also help IDCS 1426 to become as massive as it is this early.
"When it is hit by another group or cluster, the cool core will slosh around like wine in the bottom of the wine glass," Brodwin said. "Eventually it will settle towards the center, but it hasn't settled yet."
Very smooth and symmetric distribution of hot gas in the rest of the cluster point to rapid development of IDCS 1426, while the lack of elements other than hydrogen and helium reveal that it truly is a young cluster.
The study was made public at the 227th meeting of the American Astronomical Society meeting in Kissimmee, Florida, USA and will be published in Astrophysical Journal.