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Hubble Finds that Monster "El Gordo" Galaxy Cluster is Bigger than Thought

For Release: April 03, 2014

STScI

El Gordo
NASA, ESA, J. Jee (Univ. of California, Davis), J. Hughes (Rutgers Univ.), F. Menanteau (Rutgers Univ. & Univ. of Illinois, Urbana-Champaign), C. Sifon (Leiden Obs.), R. Mandelbum (Carnegie Mellon Univ.), L. Barrientos (Univ. Catolica de Chile), and K. Ng (Univ. of California, Davis)
Press Image and Caption

NASA's Hubble Space Telescope has weighed the largest known galaxy cluster in the distant universe and found that it definitely lives up to its nickname: El Gordo (Spanish for "the fat one").

By precisely measuring how much the gravity from the cluster's mass warps images of far more distant background galaxies, a team of astronomers has calculated the cluster's mass to be as much as 3 million billion times the mass of our Sun. The Hubble data show that the cluster is roughly 43 percent more massive than earlier estimates based on X-ray and dynamical studies of the unusual cluster.

"It's given us an even stronger probability that this is really an amazing system very early in the universe," said team lead James Jee of the University of California at Davis.

A fraction of this mass is locked up in several hundred galaxies that inhabit the cluster and a larger fraction is in hot gas that fills the entire volume of the cluster. The rest is tied up in dark matter, an invisible form of matter that makes up the bulk of the mass of the universe.

Though galaxy clusters as massive are found in the nearby universe, such as the so- called Bullet cluster, nothing like this has ever been seen to exist so far back in time, when the universe was roughly half of its current age of 13.8 billion years. The team suspects such monsters are rare in the early universe, based on current cosmological models.

The immense size of El Gordo was first reported in January 2012. Astronomers estimated its huge mass based on observations from NASA's Chandra X-ray Observatory and galaxy velocities measured by the European Southern Observatory's Very Large Telescope array in Paranal, Chile. They were able to put together estimates of the cluster's mass based on the motions of the galaxies moving inside the cluster and the very high temperatures of the hot gas between the cluster galaxies.

The challenge was that they noticed that the cluster (catalogued as ACT-CL J0102- 4915) looked as if it might have been the result of a titanic collision between a pair of galaxy clusters the researchers describe as "seeing two cannonballs hit each other."

"We wondered what happens when you catch a cluster in the midst of a major merger and how the merger process influences both the X-ray gas and the motion of the galaxies," explained John Hughes of Rutgers University. "So the bottom line is that because of the complicated merger state, it left some questions about the reliability of the mass estimates we were making."

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