Ancient, massive galaxy cluster

Astronomers announced this week the discovery of the most distant cluster of galaxies ever found. The cluster may also be the most massive one yet seen at such an early era in the universe.

Almost 10 billion light-years from Earth, the cluster contains hundreds of galaxies surrounded by superheated, X-ray-emitting gas at more than 10 million degrees. The XMM Cluster Survey (XCS) team used observations from the European X-ray Multi Mirror (XMM) Newton satellite to discover the cluster and then determined its distance using the 10-meter W. M. Keck telescope in Hawaii.

"I couldn't believe it when I saw that this distant cluster appears to be full of old galaxies," said lead researcher Adam Stanford, a research scientist at UC Davis and at Lawrence Livermore National Laboratory. "It's like finding a picture of your grandfather as an adult in the nineteenth century - how could he have existed so long ago?"

Stanford and his colleagues said the total mass of the cluster is enough to contain 500 trillion stars comparable in mass to our Sun. That's a surprising stellar mass for a galaxy cluster to have achieved at such an early era in the evolution of the universe, Stanford said.

"This cluster is a challenge for our models of how massive galaxies formed, and to our understanding of the way such a massive cluster can exist at a relatively early era in the universe," he said.

Using the temperature of the X-ray emitting gas, Kivanc Sabirli, a graduate student at Carnegie Mellon University, determined that the cluster, named XMMXCS 2215-1738, is approximately 500 trillion times the mass of the sun. Such massive clusters are expected to grow through the amalgamation of many smaller masses, like groups of galaxies. This process takes time, however. Most of the mass is thought to be "dark matter," a mysterious, invisible form of matter that dominates the mass of all galaxies in the universe.

Team member Robert Nichol, from the University of Portsmouth, England, said the patch of the universe covered by the Cluster Survey contains hints of more than 1,600 additional galaxy clusters waiting to be confirmed and studied in detail. "The total number of clusters depends on the amount of dark matter there is," Nichol said. "So this will give us a wonderful measure of how much dark matter there is in the universe."

"Such a massive cluster at this early time in the Universe is only expected in a flat universe full of dark energy," added Pedro Viana, a cosmologist on the team from the University of Porto in Portugal. "It is yet more evidence that we live in a strange Universe."

"It's special," Stanford said. "We will be studying this cluster using all the tools available to us. We are already getting detailed pictures using the Hubble Space Telescope."

Stanford is also a team member for a separate galaxy-cluster study that presented its results at the same meeting, the 208th American Astronomical Society meeting in Calgary. Using the Spitzer Space Telescope, the team found almost 300 galaxy clusters and smaller groups of galaxies. Nearly 100 of their finds are more than eight billion light-years away.

The work was funded by NASA, the Particle Physics and Astronomy Research Council (U.K.), the Hosie Bequest, and the National Science Foundation.

Source: Lawrence Livermore National Laboratory