South Pole Telescope finds most massive galaxy cluster to date
Posted October 22, 2010
Astronomers using the South Pole Telescope report that they have discovered the most massive galaxy cluster yet, tipping the scales at the equivalent of 800 trillion suns, and holding hundreds of galaxies.
“This galaxy cluster wins the heavyweight title. It’s among the most massive clusters ever found at this distance,” said Mark Brodwin , an astronomer at the Harvard-Smithsonian Center for Astrophysics , in a press release.
Brodwin is first author on the paper announcing the discovery, which appeared in a recent edition of the Astrophysical Journal. John Carlstrom at the University of Chicago is the lead principal investigator of the South Pole Telescope (SPT), an experiment funded by the National Science Foundation (NSF) in Antarctica.
The cluster (designated SPT-CL J0546-5345) is 7 billion light-years away, meaning scientists see it as it appeared 7 billion years ago, when the universe was half as old as now and the Earth and its solar system didn’t exist yet.
Since then, the heavyweight cluster should have grown about four times larger, making it one of the most massive galaxy clusters in the universe.
“This cluster is full of ‘old’ galaxies, meaning that it had to come together very early in the universe’s history — within the first two billion years,” Brodwin said.
Galaxy clusters like this can be used to study how dark matter and dark energy influenced the growth of cosmic structures. Dark energy is a hypothetical form of energy believed to be behind the rate of expansion of the universe. Dark matter is inferred to exist from its apparent gravitational effect on visible matter.
Long ago, when the universe was smaller and more compact, gravity had a greater influence. It was easier for galaxy clusters to grow, especially in areas that were already more than their surroundings.
As the universe expanded at an accelerating rate due to dark energy, it grew more diffuse. Dark energy now dominates over the pull of gravity and chokes off the formation of new galaxy clusters.
Brodwin and his colleagues spotted their quarry in data collected from the new SPT, which is currently completing its pioneering millimeter-wave survey of a huge swath of sky covering 2,500-square degrees.
The main goal of the SPT survey is to find a large sample of massive galaxy clusters in order to learn more about dark energy. Additional goals include understanding the evolution of hot gas within galaxy clusters, studying the evolution of massive galaxies in clusters, and identifying distant, rapidly star-forming galaxies.
The SPT hunting technique for giant galaxy clusters uses the Sunyaev-Zel’dovich (SZ) effect — a small distortion in the cosmic microwave background (a pervasive all-sky glow left over from the Big Bang ). Such distortions are created as background radiation passes through a large galaxy cluster. In 2008, the telescope detected its first galaxy clusters using the SZ effect.
Once this distant cluster was found, the team studied it with the Infrared Array Camera on the Spitzer Space Telescope to pinpoint galaxies within the cluster. Detailed observations of the galaxies’ speeds with the Magellan telescopes in Chile proved that the galaxy cluster was a heavyweight.
The team expects to find many more giant galaxy clusters lurking in the distance once the South Pole Telescope survey is completed.
“After many years of effort, these early successes are very exciting. The full SPT survey, to be completed next year, will rewrite the book on the most massive clusters in the early universe,” Brodwin said.
NSF-funded research in this story: John Carlstrom, John Ruhl, Joseph Mohr, William Holzapfel and Nils Halverson, University of Chicago, Award No. 0638937 . Adapted from a press release from the Harvard-Smithsonian Center for Astrophysics, http://www.cfa.harvard.edu/ news/2010/pr201020.html .
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