Clash of clusters provides new dark matter clue
BY DR EMILY BALDWIN
ASTRONOMY NOW

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.

Our latest 132-page Astronomy Now special edition is an extravaganza of astronomy for the year ahead, with a complete 30-page guide to observing the planets, moon, meteor showers, two solar eclipses, and the deep sky in 2010.
 U.K. STORE
 E.U. STORE
 U.S. & WORLDWIDE STORE

Hubble Reborn
Hubble Reborn takes the reader on a journey through the Universe with spectacular full-colour pictures of galaxies, nebulae, planets and stars as seen through Hubble's eyes, along the way telling the dramatic story of the space telescope, including interviews with key scientists and astronauts.
 U.K. STORE
 E.U. STORE
 U.S. & WORLDWIDE STORE

3D Universe
Witness the most awesome sights of the Universe as they were meant to be seen in this 100-page extravaganza of planets, galaxies and star-scapes, all in 3D!
 U.K. STORE
 E.U. STORE
 U.S. & WORLDWIDE STORE

This special publication features the photography of British astro-imager Nik Szymanek and covers a range of photographic methods from basic to advanced. Beautiful pictures of the night sky can be obtained with a simple camera and tripod before tackling more difficult projects, such as guided astrophotography through the telescope and CCD imaging.
 U.K. STORE
 E.U. STORE
 U.S. & WORLDWIDE STORE

Explore the Universe with these new versions of the award-winning Starry Night Software. Available now from the Astronomy Now Store.
 U.K. STORE
 E.U. STORE
 U.S. & WORLDWIDE STORE

Astronomy Now is pleased to announce the publication of Exploring Mars. The very best images of Mars taken by orbiting spacecraft and NASA's Spirit and Opportunity rovers fill up the 98 glossy pages of this special edition!
 U.K. STORE
 E.U. STORE
 U.S. & WORLDWIDE STORE

This new poster features some of the best pictures from NASA's amazing Mars Exploration Rovers Spirit and Opportunity.
 U.K. STORE
 E.U. STORE
 U.S. & WORLDWIDE STORE