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Telescope seen at twilight.
Photo Credit: Daniel Luong-Van/Antarctic Photo Library
Red lights illuminate the South Pole Telescope during the 2010 winter. Researchers using the telescope have found the most massive galaxy cluster to date, tipping the scales at 800 trillion suns.

Heavyweight discovery

South Pole Telescope finds most massive galaxy cluster to date

Astronomers using the South Pole Telescope External Non-U.S. government site 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 External Non-U.S. government site, an astronomer at the Harvard-Smithsonian Center for Astrophysics External Non-U.S. government site, 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 External Non-U.S. government site at the University of Chicago External Non-U.S. government site is the lead principal investigator of the South Pole Telescope (SPT), an experiment funded by the National Science Foundation (NSF) External U.S. government site 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.

Galaxy Cluster
Photo Credit: Infrared Image: NASA/JPL-Caltech/M. Brodwin (Harvard-Smithsonian CfA) Optical Image: CTIO Blanco 4-m telescope/J. Mohr (LMU Munich)
An infrared/optical representative-color image of a massive galaxy cluster located 7 billion light-years from Earth. Galaxies with "old" stellar populations, like modern-day ellipticals, are circled in yellow; galaxies with "young" stellar populations, like modern-day spirals, are circled in blue.
Galaxy Cluster
Photo Credit: CTIO Blanco 4-m telescope/J. Mohr (LMU Munich)
This optical image of the newfound galaxy cluster highlights how faint and reddened these galaxies are due to their great distance. The cluster remained hidden until the South Pole Telescope spotted it by looking for distortions in the cosmic microwave background. The blue streak is a satellite passing through the field of view during the timed exposure.

“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 External U.S. government site 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 External U.S. government site — a small distortion in the cosmic microwave background (a pervasive all-sky glow left over from the Big Bang External U.S. government site). 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 External Non-U.S. government site on the Spitzer Space Telescope External Non-U.S. government site 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 External U.S. government site. Adapted from a press release from the Harvard-Smithsonian Center for Astrophysics, http://www.cfa.harvard.edu/ news/2010/pr201020.html External Non-U.S. government site.

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