Astronomy image of the day: Pandora's cluster

"Pandora's Cluster" --The Most Colossal Known Galactic Collision http://www.dailygalaxy.com/my_weblog/2012/11/pandoras-cluster-the-most-colossal-known-galaxy-collision.html#more

This is the most colossal known smash-up observed in the universe. This awesome bundle of galaxies, nicknamed Pandora's cluster, turns out to be the result of a violent mashup between at least four separate galaxy clusters that occured over hundreds of millions of years. The cluster's official name is Abell 2744. Using various telescopes including NASA's Hubble Space Telescope and the European Southern Observatory's Very Large Telescope in Chile, astronomers have mapped all of its ingredients and pieced together its violent history. The image suggests that several galaxy clusters have collided over about 350 million years, with visible galaxies making up 5 per cent of the mass, while searingly hot gas that glows in X-ray making up 20 per cent. The rest is the invisible x-factor, dark matter, the mysterious "X" force that reveals itself only because its gravity bends light from background galaxies. "The complicated and uneven distribution of the different types of matter is extremely unusual and fascinating," says Dan Coe from the Space Telescope Science Institute in Baltimore, Maryland, one of the study leaders. The bright "bullet" near the cluster's core marks where gas from two clusters has collided to create a shock wave. Dense dark matter appears to congregate in one outer part of the cluster that's devoid of any galaxies or gas. Meanwhile, a lone ghostly gas clump has mysteriously detached itself and "sailed" ahead of its associated dark matter, a surprising observation that might clarify how dark matter and normal matter interact. The Daily Galaxy via NASA, ESA, ESO, CXC & D. Coe (STScI)/J. Merten (Heidelberg/Bologna)