Description of the image: This mosaic from Hubble shows the cluster called El Gordo (the big one, in Spanish). The cluster contains several hundred galaxies, but in this image, there are many more as some are seen distorted multiple times by gravitational lenses. They spread out in a circle but are actually multiple views of a single galaxy. Credit: NASA, ESA, and J. Jee (University of California, Davis).
The El Gordo galaxy cluster, officially designated ACT-CL J0102-4915, is one of the most massive and energetic galaxy clusters observed in the universe. Located approximately 7 billion light-years away, this cluster was discovered in 2012 by an international team of scientists using the Atacama Cosmology Telescope (ACT) in Chile. Its name, "El Gordo," which means "the big one" in Spanish, reflects its imposing size and immense mass, estimated at about 3×1015 solar masses. The size of El Gordo is also impressive, with a diameter of about 3 million light-years.
El Gordo is composed of thousands of galaxies, hot gas, and dark matter (observed by X-rays). About 85% of the cluster's mass is in the form of dark matter, a hypothetical form of matter that does not interact with light, but whose presence is inferred from its gravitational effects.
X-ray observations, conducted by the Chandra space telescope, reveal that the hot gas in El Gordo has an average temperature of about 14 keV (approximately 160 million degrees Kelvin). This high temperature indicates the violent dynamics of the cluster, often attributed to a collision between two smaller clusters.
One of the main methods used to study El Gordo is the Sunyaev-Zel'dovich (SZ) effect. This effect occurs when the cosmic microwave background (CMB) radiation interacts with the hot gas in the cluster, shifting the energy of the CMB photons. This interaction allows the cluster to be detected and certain properties, such as the pressure of the hot gas, to be measured.
El Gordo also acts as a gravitational lens, bending the light from objects located behind it. By observing the distortions in the images of background galaxies, astronomers can map the mass distribution in the cluster, including dark matter. This technique has been used to confirm the immense mass of El Gordo and to study its internal structure.
X-ray observations are essential for studying the hot gas in galaxy clusters. Data from the Chandra space telescope have made it possible to measure the temperature, density, and distribution of the hot gas in El Gordo, providing clues about the dynamic history of the cluster and the ongoing merging processes.
The study of El Gordo provides important tests for cosmological models. The extreme mass and size of the cluster, as well as its existence at a relatively early time in the universe, challenge some predictions of standard structure formation models. Cosmologists use clusters like El Gordo to refine the parameters of the standard cosmological model, including the density of dark matter and the nature of dark energy.
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