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.