This image combines visible light exposures of galaxy cluster Abell 2744 taken by the NASA/ESA Hubble Space Telescope and the European Southern Observatory's Very Large Telescope, with X-ray data from NASA's Chandra X-ray Observatory and a mathematical reconstruction of the location of dark matter.
Credit: NASA/ESA/ESO/CXC/STScI/Heidelberg/Bologna
PASADENA: The complex and violent history of a galaxy nicknamed Pandora's Cluster has been pieced together to reveal that it may be the result of a simultaneous pile-up of at least four separate galaxy clusters.
Using telescopes including the Hubble Space Telescope and the European Space Organisation's Very Large Telescope, scientists have identified the strange effects produced by the collision that have previously never been seen together.
"Like a crash investigator piecing together the cause of an accident, we can use observations of these cosmic pile-ups to reconstruct events that happened over a period of hundreds of millions of years," said one of the lead scientists Julian Merten from the Institute of Theoretical Astrophysics at the University of Heidelberg in Germany.
"This can reveal how structures form in the universe, and how different types of matter interact with each other when they are smashed together."
Treasure trove of info
When huge clusters of galaxies crash together, the resulting mess is a treasure trove of information for astronomers.
By investigating one of the most complex and unusual colliding clusters in the sky, an international team of astronomers has pieced together the history of a cosmic crash that took place over a period of 350 million years.
The galaxy Abell 2744 has now been studied in more detail than ever before by combining data from the NASA/ESA Hubble Space Telescope, ESO's Very Large Telescope (VLT), the Japanese Subaru telescope and NASA's Chandra X-Ray Observatory.
Phenomena never seen before
"We nicknamed it Pandora's Cluster because so many different and strange phenomena were unleashed by the collision. Some of these phenomena had never been seen before," said Renato Dupke from the University of Michigan, another member of the team.
The galaxies in the cluster are clearly visible in the Hubble and VLT images. Although the galaxies are bright they make up less than 5% of the mass there.
The rest is gas (around 20%), which is so hot that it shines only in X-rays, and dark matter (around 75%), which is completely invisible. To understand what was going on in the collision the team needed to map the positions of all three types of matter in Abell 2744.
Showing components coming together
Dark matter is particularly elusive as it does not emit, absorb or reflect light, but only makes itself apparent through its gravitational attraction. To pinpoint the location of this mysterious substance the team exploited a phenomenon known as gravitational lensing.
This is the bending of light rays from distant galaxies as they pass through the gravitational field present in the cluster. The result is a series of telltale distortions in the images of galaxies in the background of the Hubble and VLT observations. By carefully plotting the way that these images are distorted, it is possible to map quite accurately where the mass - and hence the dark matter - actually lies.
By comparison, finding the hot gas in the cluster is simpler as NASA's Chandra X-ray Observatory can observe it directly. These observations are not just crucial to find out where the gas is, but also to show the angles and speeds at which different components of the cluster came together.
