El Gordo - A "Fat" Distant Galaxy Cluster
The newly discovered galaxy cluster  has been nicknamed El Gordo - the "big" or "fat one" in Spanish. It consists of two separate galaxy subclusters colliding at several million kilometres per hour, and is so far away that its light has travelled for seven billion years to reach the Earth.
"This cluster is the most massive, the hottest, and gives off the most X-rays of any cluster found so far at this distance or beyond," said Felipe Menanteau of Rutgers University, who led the study. "We devoted a lot of our observing time to El Gordo, and I'm glad our bet paid off and we found an amazing cluster collision."
Clusters of galaxies are the largest objects in the Universe held together by gravity. The process of their formation, when smaller groups of galaxies merge together, very much depends on the amount of dark matter and dark energy in the Universe at that time - so studying clusters can shed some light on these mysterious components of the cosmos.
"Gigantic galaxy clusters like this one are just what we were aiming to find," said team member Jack Hughes, also of Rutgers. "We want to see if we understand how these extreme objects form, using the best models of cosmology that are currently available."
The team, led by Chilean and Rutgers astronomers, found El Gordo by detecting a distortion of the cosmic microwave background radiation. This faint glow is the remnant of the first light from the Big Bang, the extremely hot and dense origin of the Universe about 13.7 billion years ago. This radiation left over from the Big Bang interacts with electrons in the hot gas in galaxy clusters, distorting the appearance of the background glow seen from Earth . The denser and bigger the cluster, the bigger this effect. El Gordo was picked up in a survey of the microwave background with the Atacama Cosmology Telescope .
ESO's Very Large Telescope was used by the team to measure the velocities of the galaxies in this huge cluster collision and also to measure its distance from Earth. In addition NASA's Chandra X-ray Observatory was used to study the hot gas in the cluster.
Although a cluster of El Gordo's size and distance is very rare, the authors say that the new results are still consistent with astronomers' current understanding of a Universe that started with a Big Bang and is mostly made of dark matter and dark energy.
El Gordo most probably formed just like the Bullet Cluster, a spectacular interacting cluster of galaxies that is almost four billion light-years closer to Earth. In both clusters there is evidence that normal matter, mainly composed of hot, X-ray-bright gas, has been wrenched apart from the dark matter. The hot gas was slowed down by the collision, but the dark matter was not.
"This is the first time we've found a system like the Bullet Cluster at such a large distance," said Cristóbal Sifón, student at the Pontificia Universidad Católica de Chile (PUC) in Santiago. "It's like the old saying: If you want to understand where you're going, you have to know where you've been."
 The formal name of the cluster is ACT-CL J0102-4915, the first part of the name shows that it is a galaxy cluster found using data from the Atacama Cosmology Telescope and the second part indicates the location of the object on the sky, in the southern constellation of Phoenix.
 The effect is called the Sunyaev-Zel'dovich (SZ) effect after the Russian astronomers, Rashid Sunyaev and Yakov Zel'dovich who predicted it in the late 1960s.
 The Atacama Cosmology Telescope (ACT) is a six-metre telescope on Cerro Toco in the Atacama Desert in the north of Chile and close to the site of ALMA. It is designed to make high-resolution surveys of the microwave sky to study the cosmic microwave background radiation.
These results on El Gordo are being announced on 10 January 2012 at the 219th meeting of the American Astronomical Society in Austin, Texas. A paper, "The Atacama Cosmology Telescope: ACT-CL J0102−4915 'El Gordo', A Massive Merging Cluster at Redshift 0.87" by Felipe Menanteau et al, describing these results has been accepted for publication in The Astrophysical Journal.
The team consists of: Felipe Menanteau (Rutgers University, USA), John P. Hughes (Rutgers), Crisóbal Sifón (Pontificia Universidad Católica de Chile [PUC]), Matt Hilton (University of Nottingham, UK), Jorge González (PUC), Leopoldo Infante (PUC), L. Felipe Barrientos (PUC) , Andrew J. Baker (Rutgers) , Sudeep Das (University of California, Berkeley, USA; Princeton University, USA), Mark J. Devlin (University of Pennsylvania, USA), Joanna Dunkley (Oxford University, UK) , Adam D. Hincks (Princeton University), Arthur Kosowsky (University of Pittsburgh, USA) , Danica Mardsen (University of Pennsylvania), Tobias A. Marriage (The Johns Hopkins University, Baltimore, USA) , Kavilan Moodley (University of KwaZulu-Natal, Durban, South Africa), Michael D. Niemack (NIST, Boulder, USA) , Lyman A. Page (Princeton University) , Erik D. Reese (University of Pennsylvania) , Neelima Sehgal (Stanford University, USA), Jon Sievers (University of Toronto, Canada) , David N. Spergel (Princeton University), Suzanne T. Staggs (Princeton University) and Edward Wollack (Goddard Space Flight Center, USA).
The year 2012 marks the 50th anniversary of the founding of the European Southern Observatory (ESO). ESO is the foremost intergovernmental astronomy organisation in Europe and the world's most productive astronomical observatory. It is supported by 15 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world's most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world's largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning a 40-metre-class European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become "the world's biggest eye on the sky".
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