This composite image shows the massive galaxy cluster MACSJ0717.5+3745 (MACSJ0717, for short), where four separate galaxy clusters have been involved in a collision, the first time such a phenomenon has been documented. Hot gas is shown in an image from NASA's Chandra X-ray Observatory and galaxies are shown in an optical image from NASA's Hubble Space Telescope. The hot gas is color-coded to show temperature, similar to a temperature map of the Earth given in a weather forecast. In MACSJ0717 the coolest gas is shown as reddish purple, the hottest gas is blue and the temperatures in between are purple.


The repeated collisions in MACSJ0717 are caused by a 13-million-light-year-long stream of galaxies, gas, and dark matter - known as a filament -- pouring into a region already full of matter. A collision between gas in two or more clusters causes the hot gas to slow down. However, the galaxies, which are mainly empty space, do not slow down as much and so they move ahead of the gas. Therefore, the speed and direction of each cluster's motion -- perpendicular to the line of sight -- can be estimated by studying the offset between the average position of the galaxies and the peak in the hot gas.

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Carnival of Space

-Kimberly Arcand, CXC

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After studying Xray images of dozens of galaxy clusters (most by CXO or XMM), I’ve yet to see such a bizarre pattern as in MACSJ0717. The paper points out that this is the confluence of 4! galaxy clusters mixing it up (See original paper at: Those downloading the highest-res optical images, especially the cropped image, will be able to make out numerous blue arcs in the vicinity of the densest portion of this amazing cluster. These represent distant galaxies lensed by the gravitational potential of the cluster(s) imaged by Hubble. Reference is made to a previously known galaxy ‘filament’ that was discovered in an earlier study. That paper, with some awesome imagery from the Subaru scope, can be found here:…609L..49E. What an incredible coincidence to be able to see 4 galaxy clusters caught in gravitational embrace!

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