Mapping Dark Matter In Distant Galaxy Cluster

(CORDIS) — Two teams of astrophysicists from Denmark, Israel, France and the United States have used data from several leading telescopes to map the distribution of dark matter in a galaxy cluster known as Abell 383, located about 2.3 billion light years from Earth. As well as successfully noting where the dark matter lies in the two dimensions across the sky, the teams also managed to determine how the dark matter is distributed along the line of sight.

Astrophysicists have long tried to understand the nature of dark matter. Although it is detectable through its gravitational effects, dark matter is invisible material that does not emit or absorb any type of light; there is roughly six times as much dark matter as there is ‘normal’ matter in the Universe. Galaxy clusters are the largest gravitationally bound structures in the Universe; they play an important role in research on dark matter. But in order to analyse galaxy clusters, astrophysicists need to be able to accurately determine the three-dimensional (3D) structures and masses of clusters. It is at the centre of galaxy clusters that dark matter can be found in its highest concentration.

In the two studies, published in the Astrophysical Journal and the Monthly Notices of the Royal Astronomical Society, the teams provide the most detailed 3D pictures taken to date of dark matter in a galaxy cluster. Their findings show that the dark matter stretches out like a gigantic football, rather than being spherical like a basketball, and that the point of the football is aligned close to the line of sight. They also note that hot gas is by far the dominant type of normal matter in the cluster.

The teams’ research technique involved combining X-ray observations of the ‘normal matter’ in the cluster with gravitational lensing information determined from optical data. Gravitational lensing causes the material in the galaxy cluster, both normal and dark matter, to bend and distort the optical light from background galaxies. The distortion is severe in some parts of the image, producing an arc-like appearance for some of the galaxies. In other parts of the image, the distortion is subtle; statistical analysis is used to study the distortion effects and probe the dark matter.

The European team concluded that the increased concentration of the dark matter toward the centre of the cluster is in agreement with most theoretical simulations.

However, the American team found evidence that the amount of dark matter is not peaked as dramatically toward the centre as the standard cold dark matter model predicts. Their paper describes this as being the ‘most robust case yet’ made for such a discrepancy with the theory.

The contrasting conclusions reached by the two teams most likely stem from differences in the data sets and the detailed mathematical modelling used. One important difference is that because the European team used velocity information in the central galaxy, they were able to estimate the density of dark matter at distances that approached as close as only 6 500 light years from the centre of the cluster. The American team did not use velocity data and their density estimates were unable to approach as close to the cluster’s centre, reaching to within 80 000 light years.

The data used in the two studies was pooled from NASA’s Chandra X-ray Observatory, the Hubble Space Telescope (HST), the Very Large Telescope, and the Sloan Digital Sky Survey, and the Japanese Subaru Telescope.