A monstrously huge cluster of galaxies lurks 7 billion light-years away. The cluster weighs in around 800 trillion suns and holds hundreds of galaxies, making it the most massive galaxy cluster ever found at such a great distance.
Despite its tremendous bulk, the cluster was hidden until astronomers looked for the distortions it created in the cosmic microwave background, the oldest light in the universe. This light was emitted when ions and electrons first combined to form atoms just after the Big Bang, and has been traveling through the rest of the matter in the universe for the last 13.7 billion years or so to reach telescopes on Earth. As the light passes through massive galaxy clusters, it can get distorted in a phenomenon called the Sunyaev-Zel'dovich effect.
Astronomers using the South Pole Telescope at (where else?) the South Pole have found several galaxy clusters by searching for this effect. But this newest cluster, named SPT-CL J0546-5345, is the heavyweight champion.
Because the cluster is so far away, astronomers see it as it appeared 7 billion years ago, when the universe was half its current age and before the solar system even existed. Even then, the cluster was almost as massive as the nearby Coma Cluster of galaxies, which is one of the densest known. Since then, the cluster should have quadrupled in size, astronomers inferred, making it one of the most massive clusters in the universe. Details on the cluster are published in the Astrophysical Journal.
The cluster also appears full of "old" galaxies that are not forming many stars at a rapid rate, which means the galaxies must have come together to form the cluster within the first 2 billion years of the universe's history. In the image above, a follow-up with the infrared Spitzer Space Telescope and an optical camera on the Blanco 4-meter telescope at the Cerro Tololo Inter-American Observatory in Chile, galaxies with "old" stellar populations are circled in yellow, and galaxies with "young" stellar populations are circled in blue.
Galaxy clusters this massive and distant can be used to study how dark matter and dark energy influenced the growth of cosmic structures.
Image: Infrared: NASA/JPL-Caltech/M. Brodwin (Harvard-Smithsonian CfA) Optical: CTIO Blanco 4-m telescope/J. Mohr (LMU Munich)