MACS J0717.5+3745 (MACS J0717 or MACS 0717 for short) is a cluster of galaxies
located 5.4 billion light years from the Earth in the constellation Auriga.
MACS J0717.5+3745 (Cluster of Galaxies) : Picture
Galaxy Cluster MACS J0717
(C) NASA, ESA, CXC, C. Ma, H. Ebeling, and E. Barrett (University of Hawaii/IfA),
et al., and STScI
This composite image shows the massive galaxy cluster MACS J0717.5+3745 (MACS J0717, 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, where the coolest gas is reddish purple, the hottest gas is blue, and the temperatures in between are purple.
The repeated collisions in MACS J0717 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 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, 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.
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.
Compass and Scale Image of MACS J0717
(C) NASA, ESA, and Z. Levay (STScI)
(C) NASA, ESA, and H. Ebeling (University of Hawaii)
In this episode of the Hubblecast, Joe Liske (aka Dr J) shows how a team of astronomers has used Hubble and a battery of other telescopes to discover the secrets of massive galaxy cluster MACS J0717. They have found that an invisible filament of dark matter extends out of the cluster. This is our first direct glimpse of the shape of the scaffolding that gives the Universe its structure.
The filament extending from MACS J0717 (artist's impression)
This video shows a computer simulation of the dark matter filament's shape as it extends back from the cluster into the background.
Note that while Hubble's determination of the 2D position of the galaxies is very accurate, measurements of distance in astronomy always contain a degree of uncertainty. Due to the scatter inherent in these measurements, the length and orientation of the filament as calculated by the scientists is derived from a statistical analysis -- the accuracy of any individual galaxy's location is quite low.
For the purposes of this illustration, the perspective has been slightly compressed to reduce the scatter in the galaxies' locations, while the dark matter filament is an artist's impression based on the scientists' models of the filament's structure.