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El Gordo Galaxy Lives Up To Its Name

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Justine Alford

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606 El Gordo Galaxy Lives Up To Its Name
Galaxy cluster "El Gordo". NASA, ESA, J. Jee (University of California, Davis)

The largest known galaxy cluster in the distant universe has been weighed using NASA’s Hubble Space Telescope and was found to be over 40 percent larger than earlier estimates which used a different technique.

The cluster’s official name is ACT-CL J0102-4915, but has been aptly nicknamed “El Gordo” which is Spanish for “the fat one”. It is composed of several hundred galaxies and is thought to be approximately 9.7 billion light years away from earth. Although similarly large clusters are known to exist, it is rare to find them in the early universe like El Gordo.


Its incredible mass was first reported in January 2012 using observations from the European Southern Observatory’s Very Large Telescope array and NASA’s Chandra X-ray observatory. This allowed an estimation of the mass of the cluster from measurements of both the velocity of the galaxies moving within the cluster and the temperatures of gas residing between the cluster galaxies. The story was complicated, however, by observations that seemed to suggest that El Gordo was the result of a collision between two galaxy clusters. According to John Hughes from Rutgers University, this raised the possibility that the process of collision may have affected what they were measuring, suggesting that the original estimates may have been unreliable.

In an attempt to obtain a more reliable measurement, scientists turned to data collected from Hubble. They measured the extent of image distortion of more distant background galaxies caused by gravity from the mass of the cluster, which is called “weak lensing”. The more the image was distorted, or warped, the greater the mass of the cluster. They discovered that the cluster had a mass 3 million billion times that of our Sun, and is 43 percent larger than the earlier estimates.

The cluster is so large that scientists will next start working on constructing a mosaic image to get a more complete picture, since it is too big to fit in Hubble’s field of view.


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