By Paul Rincon
Science reporter, BBC News, Austin
Abell 901/902 is one of the biggest structures in the Universe
Astronomers have revealed the effects of unseen dark matter as it tugs on galaxies in a crowded supercluster.
Dark matter acts as invisible cosmic "scaffolding" upon which visible stars and galaxies are assembled.
The dark matter in this instance has pooled into four dense clumps, in which hundreds of old galaxies are embedded.
Using the Hubble Space Telescope, a team of scientists mapped the dark matter at a better resolution than has ever been achieved before.
Co-author Catherine Heymans, of the University of British Columbia in Canada, said the survey covered one of the largest patches of sky to be observed by Hubble.
Through the lens
Dark matter does not reflect or emit detectable light, yet it accounts for most of the mass in the Universe. The researchers were able to detect it indirectly using a technique called weak gravitational lensing.
To reach us, the light from galaxies has to pass through intervening dark matter.
This dark material bends light in much the same way as light is bent when travelling through a lens.
Details of the research on the supercluster Abell 901/902 were released at the 211th meeting of the American Astronomical Society in Austin, Texas.
A supercluster is a giant cosmic structure consisting of groups of galaxies and smaller clusters. The Stages (Space Telescope Abell 901/902 Galaxy Evolution Survey) project was also able to resolve detailed structures for the visible galaxies embedded in the cosmic web of dark matter.
The galaxies in Abell 901/902 bear the scars of a violent migration from the outskirts of the giant cosmic structure to the dense regions.
HUBBLE SPACE TELESCOPE
Named after the great US astronomer Edwin Hubble
Launched in 1990 into a 600km-high circular orbit
Equipped with a 2.4m primary mirror and five instruments
Length: 15.9m; diameter: 4.2m; Mass: 11,110kg
The study illustrates the way that galaxies are drawn into larger and larger groups and clusters by the inevitable force of gravity as the Universe evolves.
Astronomers have known for a long time that galaxies in crowded environments tend to be older, redder and rounder than ones elsewhere.
As these galaxies made their passage into the densest cluster cores, they have been subjected to extreme violence.
They experience high-speed collisions with other galaxies, are stripped of their gas - the fuel supply they use to make new stars - and become distorted due to the strong gravitational pull of the underlying, invisible dark matter.
"Any or all of these effects may play a role in the transformation of galaxies, which is what we're trying to determine," said co-author Meghan Gray, from the University of Nottingham, UK.
What is new about the latest findings is that they pinpoint where most of the action is taking place.
"From a number of lines of evidence, we are seeing that the outskirts of the galaxy clusters are where the real transformations are happening, and we have seen some of these transformations in progress," she observed.
"As these galaxies are moving from the country to the big city, it is in the suburbs that we see them in the process of changing."
Astronomers say this is because once the galaxies reach the dense centres, they are moving too fast to collide and merge with other objects. While in the outskirts, their pace is more sedate, allowing different objects time to interact.
Abell 901/902 is one of the biggest structures in the Universe. It lies 2.6 billion light-years away, measures 60 million light-years across and contains about 1,000 galaxies.
An image of the Abell 901/902 supercluster taken by Hubble
The picture also uses observations from a telescope at La Silla, Chile
The magenta clumps reveal the dark matter's distribution
The supercluster's galaxies lie within the clumps of dark matter