A photo of the galaxy cluster ACT-CL J0102-4915 captured by the Hubble Space Telescope provides a small idea of the vastness of this set of galaxies that has a total mass estimated at 3 million billion times the Sun’s. For this reason it’s been nicknamed “El Gordo”, which in Spanish means “the fat one”. It intensly emits X-rays and that’s another reason of interest for astronomers that led to observe it, discovering that it’s actually formed by two smaller – or less enormous – galaxy clusters that collided.
Approximately 7 billion light from the Milky Way, the galaxy cluster ACT-CL J0102-4915, also known as SPT-CL J0102-4915, is so far away that it was discovered only in 2011 despite its vastness. It’s composed of several hundred galaxies with a total mass that is about 3,000 times the Milky Way’s. Estimates of the mass of El Gordo were made based on its gravitational lensing effects, the distortion of the space caused by the mass of the galaxies that compose it, which can be exploited to estimate it.
It took observations made with the Very Large Telescope, NASA’s Chandra X-ray Observatory and the Atacama Cosmology Telescope, instruments sensitive to different electromagnetic wavelengths, to understand that El Gordo is composed of two galaxy clusters that are colliding at a speed of millions of miles per hour. In essence, it’s the largest galaxy cluster known at those distances while there are some equally massive much closer but El Gordo is the result of a merger.
The X-ray emissions allowed to estimate the presence of normal matter and consequently to try to understand also how much dark matter is present in this double galaxy cluster. In the image below, the blue color was used to map dark matter. This work adds data to study the problem of dark matter, also testing possible alternative models proposed by scientists who don’t believe in its existence.
The image above was taken using the Hubble Space Telescope’s Advanced Camera for Surveys (ACS) and Wide-Field Camera 3 (WFC3) instruments for the RELICS (Reionization Lens Cluster Survey) program, which also aims to study dark matter. Other purposes are the study of distant galaxies, especially in clusters such as El Gordo even if they’re normally much smaller, for future studies with the James Webb Space Telescope.