Hubble weighs “the fat one”

When astronomical objects are named, astronomers like to pick out notable features for inspiration — for example, the Whirlpool Galaxy with its pinwheeling arms, or the Needle Galaxy, which appears as a long, thin streak of silver across the sky.

This image shows a galaxy cluster known as El Gordo, or “the fat one”, a very distant object that lies some ten billion light-years away from us. This grouping of galaxies certainly lives up to its nickname; it is the largest known galaxy cluster in the distant Universe and contains several hundred galaxies. What’s more, new NASA/ESA Hubble Space Telescope observations show that it is actually some 43 percent heavier than previously thought, with a mass some three million billion times the mass of the Sun — which is 3 000 times the mass of our own galaxy, the Milky Way. A small fraction of the cluster’s immense mass is locked up in the galaxies that inhabit it and a larger fraction is held in hot gas that fills its entire volume, but the majority is made up of the infamous and invisible dark matter. The location of this dark matter is mapped out in the blue overlay.