Posted: August 28, 2008
A dramatic collision between galaxy clusters captured by the Hubble and Chandra space telescopes provides striking evidence for the existence of dark matter as it separates from ordinary matter.
MACSJ0025 formed after an enormously energetic collision between two large galaxy clusters, each one thousand million million times the mass of our Sun, crashing together at millions of kilometres per hour. After the smash, the stars and hot gas in the two clusters slowed down, but the dark matter component sailed right through, allowing astronomers to study the behaviour of the different components.
MACSJ0025.4-1222, a cluster showing a clear separation between dark and ordinary matter. The blue cloud-shaped parts flanking the centre show the position of dark matter, mapped by the Advanced Camera for Surveys onboard the NASA/ESA Hubble Space Telescope. The pink middle indicates ordinary matter, charted by NASA’s Chandra X-Ray Observatory. Image: X-ray(NASA/CXC/Stanford/S.Allen); Optical/ Lensing(NASA/ STScI/UC Santa Barbara/M.Bradac).
Using visible-light images from Hubble and X-ray images from Chandra, astronomers were able to infer the total mass distribution of dark matter (coloured in blue in the image) and ordinary matter (coloured in pink), which, mostly in the form of hot gas, glows brightly in X-rays. The dark matter was detected indirectly by the gravitational lensing technique, a method that uses the distortion that mass causes as light passes by another object between an observer and background objects. Dark matter cannot be directly seen but it has mass and thus gravitational pull on the clusters’ galaxies.
The separation between the ordinary and dark material not only provides observational evidence for dark matter, but supports the idea that dark matter particles interact with each other only very weakly or not at all, apart from the pull of gravity. The observations also provide independent confirmation of a similar effect detected two years ago in a target nicknamed the Bullet Cluster, showing that this original observation was not an exception or the product of some unknown error.
The collision of galaxies in the Bullet cluster created a bow-shaped shock wave near the right side of the cluster as 70 million degree Celsius gas in the sub-cluster plowed through 100 million degree Celsius gas in the main cluster. Technically the name Bullet cluster refers to the smaller subcluster, which is moving away from the larger one. Image credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/ U.Arizona/D.Clowe et al.
There is one major difference between the Bullet Cluster and MACSJ0025, however, in that MACSJ0025 does not contain a ‘bullet’ of X-ray bright gas powering through the cluster. Nonetheless, the energies of the two collisions are comparable.
One of the great accomplishments of modern astronomy has been to establish a complete inventory of the matter and energy content of the Universe. The so-called dark matter makes up approximately 23 percent of this content, five times more than the ordinary matter that can be detected by telescopes. The latest results with MACSJ0025 once again support the case for the existence of dark matter.