Galaxy Cluster ‘El Gordo’ with Mass Map and X-ray

This is a Hubble Space Telescope image of the most massive cluster of galaxies ever seen to exist when the universe was just half of its current age of 13.8 billion years. The cluster, catalogued as ACT-CL J0102-4915, contains several hundred galaxies swarming around under a collective gravitational pull. The total mass of the cluster, as refined in new Hubble measurements, is estimated to weigh as much as 3 million billion stars like our Sun (about 3,000 times the mass of our own Milky Way galaxy) – though most of the mass is hidden away as dark matter. The location of the dark matter is mapped out in the blue overlay. Because dark matter doesn't emit any radiation, Hubble astronomers instead precisely measure how its gravity warps the images of far background galaxies like a funhouse mirror. This allowed them to come up with a mass estimate for the cluster. The cluster was nicknamed El Gordo (Spanish for "the fat one") in 2012 when X-ray observations (shown in pink) and kinematic studies first suggested it was unusually massive for the time in the early universe when it existed. The Hubble data have confirmed that the cluster is undergoing a violent merger between two smaller clusters.

About the colours

This composite image includes exposures acquired by the ACS instruments on the Hubble Space Telescope. Several filters were used to sample broad wavelength ranges. The color results from assigning different hues (colors) to each monochromatic (grayscale) image associated with an individual filter. In this case, the assigned colors are: Blue: F625W (r) Green: F775W (i) Red: F850LP (z) Cyan: Derived Mass Pink: Chandra X-ray

About the data

The Hubble data are from proposal 12755 : J. Hughes (Rutgers University), F. Menanteau (Rutgers University/University of Illinois, Urbana-Champaign), L. Infante and L.F. Barrientos (Ponticia Universidad Catolica de Chile), and R. Mandelbaum (Carnegie Mellon University), and archival data from proposal 12477 : F. High (University of Chicago) et al.