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| El Gordo | |
|---|---|
| Name | El Gordo |
| Constellation | Phoenix |
| Redshift | 0.87 |
| Mass | ~3×10^15 solar masses |
| Discovery | 2012 |
| Other names | ACT-CL J0102-4915 |
El Gordo El Gordo is a massive galaxy cluster discovered in 2012, notable for its extreme mass, high redshift, and strong gravitational lensing. It has been the subject of multiwavelength campaigns involving instruments and observatories such as the Atacama Cosmology Telescope, Chandra X-ray Observatory, Hubble Space Telescope, South Pole Telescope, and Spitzer Space Telescope. Studies of El Gordo intersect research by teams from institutions including Harvard–Smithsonian Center for Astrophysics, Max Planck Society, European Southern Observatory, NASA, and CONICYT.
El Gordo, catalogued as ACT-CL J0102-4915, lies in the Phoenix constellation and exhibits properties of a post-collision, merging system. It was identified via the Sunyaev–Zel'dovich effect by the Atacama Cosmology Telescope and confirmed by follow-up observations from Chandra X-ray Observatory and optical imaging from the Hubble Space Telescope. The cluster's extreme mass and redshift make it relevant to observational programs associated with the Lambda-CDM model, tests against alternative cosmological scenarios explored by groups at Princeton University, Stanford University, and University of Cambridge.
El Gordo was first detected in a Sunyaev–Zel'dovich survey by the Atacama Cosmology Telescope team and reported in a 2012 paper authored by members affiliated with Harvard University, University of Chicago, and University of KwaZulu-Natal. Subsequent X-ray confirmation came from observations with the Chandra X-ray Observatory that revealed high temperature plasma and merger-driven shock fronts similar to features seen in Bullet Cluster studies by researchers at University of California, Santa Cruz and Massachusetts Institute of Technology. Optical and near-infrared imaging from the Hubble Space Telescope and Spitzer Space Telescope provided redshift confirmation and magnified background sources studied by investigators from Space Telescope Science Institute and European Space Agency teams. Radio follow-up using the Australia Telescope Compact Array and millimeter observations from the Atacama Large Millimeter/submillimeter Array further characterized nonthermal emission and Sunyaev–Zel'dovich signatures pursued by groups at CSIRO and National Radio Astronomy Observatory.
El Gordo exhibits a bimodal mass distribution consistent with a major merger between two subclusters, showing high-temperature intracluster medium (ICM) plasma detected by Chandra X-ray Observatory and spectroscopic confirmation of member galaxies via instruments on the Very Large Telescope and Magellan Telescopes. Measured properties include X-ray temperatures comparable to those in clusters studied by teams at Lawrence Berkeley National Laboratory and mass estimates rivaling the most massive systems like Abell 1689 and MACS J0717.5+3745. The system displays shock fronts, radio relics, and disturbed morphology that resemble merger features analyzed in the context of collision simulations from Max Planck Institute for Astrophysics and Princeton University groups.
El Gordo's inferred total mass, derived from Sunyaev–Zel'dovich, X-ray, and gravitational lensing analyses conducted by collaborations including ACT Collaboration, SPT Collaboration, and teams from Johns Hopkins University, challenges expectations for the abundance of such massive clusters at redshift ~0.87 under the canonical Lambda-CDM model. Mass estimation techniques applied involve hydrostatic equilibrium assumptions critiqued in works from University of Michigan and weak and strong lensing reconstructions implemented by researchers at University of California, Berkeley and Durham University. Statistical studies comparing El Gordo to cluster mass functions from surveys like the Planck catalog and ROSAT All-Sky Survey have motivated theoretical examinations by groups at University of Pennsylvania and University of Oxford.
Strong gravitational lensing by El Gordo produces multiple arcs and magnified background galaxies identified in Hubble Space Telescope imaging analyzed by teams at Space Telescope Science Institute and University of Toronto. Lensing models, developed using software frameworks from Kavli Institute for Particle Astrophysics and Cosmology and Instituto de Astrofísica de Canarias, enabled reconstruction of source-plane morphologies and facilitated spectroscopic follow-up with Keck Observatory and Gemini Observatory to study high-redshift galaxies, including targets comparable to those in surveys by COSMOS and CANDELS. Lensing constraints have been combined with weak lensing shear measurements from Subaru Telescope data processed by researchers at National Astronomical Observatory of Japan.
El Gordo has been observed across the electromagnetic spectrum: millimeter-wave SZ mapping by the Atacama Cosmology Telescope and South Pole Telescope; X-ray spectroscopy by Chandra X-ray Observatory and XMM-Newton; optical/near-infrared imaging by the Hubble Space Telescope, Spitzer Space Telescope, and ground-based facilities including the Very Large Telescope and Magellan Telescopes; and radio studies with the Australia Telescope Compact Array and Giant Metrewave Radio Telescope. Teams from NASA, European Space Agency, National Science Foundation, and national observatories collaborated to interpret thermal and nonthermal signals, elemental abundances in the ICM, and star formation rates in member galaxies, linking results to surveys such as SDSS and 2MASS.
Numerical simulations and semi-analytic models addressing El Gordo's origin have been produced by research groups at Max Planck Institute for Astrophysics, Princeton University, Lawrence Livermore National Laboratory, and CITA. Simulations incorporate cosmological initial conditions consistent with WMAP and Planck parameter sets to reproduce high-velocity mergers, shock heating, and dark matter–baryon separation akin to features observed in the Bullet Cluster. Alternative formation scenarios examined by teams at University of Tokyo and University of Cambridge explore assembly bias and non-Gaussian initial conditions constrained by analyses from Fermi Gamma-ray Space Telescope and large-scale structure surveys like BOSS.