Galaxy cluster MACS J0717

This composite image shows the massive galaxy cluster MACS J0717.5+3745 (MACS J0717, for short), where four separate galaxy clusters were involved in a collision — the first time such a phenomenon has been documented. NASA's Chandra X-ray Observatory mapped hot gas, while NASA’s Hubble Space Telescope captured an optical image of the galaxies. The coolest gas is reddish purple, while the hottest gas is blue, and temperatures in between are purple. The repeated collisions in MACS J0717 are the result of a 13-million-light-year-long stream of galaxies, gas, and dark matter — known as a filament — pouring matter into a region already full of matter. A collision between the gas in two or more clusters causes the hot gas to slow down. However, the massive and compact galaxies do not slow down as much as the gas does, and so move ahead of it. Therefore, astronomers can estimate the speed and direction of each cluster's motion — perpendicular to the line of sight — by studying the offset between the average position of the galaxies and the peak in the hot gas. MACS J0717 is located about 5.4 billion light-years from Earth. It is one of the most complex galaxy clusters ever seen. Other well-known clusters, like the Bullet Cluster and MACS J0025.4-1222, involve the collision of only two galaxy clusters and show much simpler geometry.