El Gordo Galaxy Cluster

A team of astronomers using the Atacama Cosmology Telescope (ACT) in northern Chile has discovered an exceptionally massive cluster of galaxies some 7 billion light years from Earth--the largest known in the distant universe. The cluster was nicknamed El Gordo meaning the "fat one" in Spanish.

El Gordo gives us a picture of the universe at roughly half its current age and is a gold mine for studying the process of cosmic structure formation, as well as possibly shedding light on the mysterious dark matter and dark energy.

The massive cluster of galaxies was found using the Sunyaev-Zel'dovich effect in which a cluster of galaxies containing a large amount of hot gas creates a tiny temperature shift in the cosmic microwave background radiation passing through it. Galaxy clusters are the largest objects in the universe that are held together by gravity. Because their formation process depends on the amount of dark matter and dark energy in the universe, researchers use clusters to study these mysterious phenomena. El Gordo, which gives off the most X-rays of any known cluster at its distance and beyond, appears to be formed by two smaller clusters in the process of merging.

To determine El Gordo's distance and mass, team members observed almost 90 individual cluster members using the European Southern Observatory's Very Large Telescope in Chile. Separate measurements of El Gordo were made using the Spitzer satellite to show that only 1 percent of the mass was in stars as opposed to gas. To directly observe the hot gas and get an independent measurement of the mass, El Gordo was observed using the Chandra X-ray satellite.