An Intergalactic Heavyweight

 Abell 901/902

Credit: ESO

This deep-field image shows what is known as a supercluster of galaxies — a giant group of galaxy clusters which are themselves clustered together. This one, known as Abell 901/902, comprises three separate main clusters and a number of filaments of galaxies, typical of such super-structures. One cluster, Abell 901a, can be seen above and just to the right of the prominent red foreground star near the middle of the image. Another, Abell 901b, is further to the right of Abell 901a, and slightly lower. Finally, the cluster Abell 902 is directly below the red star, towards the bottom of the image.   

The Abell 901/902 supercluster is located a little over two billion light-years from Earth, and contains hundreds of galaxies in a region about 16 million light-years across. For comparison, the Local Group of galaxies — which contains our Milky Way among more than 50 others — measures roughly ten million light-years across.

This image was taken by the Wide Field Imager (WFI) camera on the MPG/ESO 2.2-metre telescope, located at the La Silla Observatory in Chile. Using data from the WFI and from the NASA/ESA Hubble Space Telescope, in 2008 astronomers were able to precisely map the distribution of dark matter in the supercluster, showing that the clusters and individual galaxies which comprise the super-structure reside within vast clumps of dark matter. To do this, astronomers looked at how the light from 60 000 faraway galaxies located behind the supercluster was being distorted by the gravitational influence of the dark matter it contains, thus revealing its distribution. The mass of the four main dark matter clumps of Abell 901/902 is thought to be around ten trillion times that of the Sun.

The observations shown here are part of the COMBO-17 survey, a survey of the sky undertaken in 17 different optical filters using the WFI camera. The COMBO-17 project has so far found over 25 000 galaxies.

Links

• The COMBO-17 survey at the Max-Planck-Institut für Astronomie, Heidelberg
• A wider-field view of the area around the Abell 901/902 supercluster

Source: ESO

Mapping Dark Matter in Galaxies

Abell 901/902

Credit: ESO

A multitude of faint galaxies, small luminous dots scattered over the dark sky, was captured by the Wide Field Imager on the MPG/ESO 2.2-metre telescope at the La Silla Observatory in Chile. Images such as this one are powerful tools to understand how dark matter is distributed in galaxies.

The picture is part of the COMBO-17 survey (Classifying Objects by Medium-Band Observations in 17 Filters), a project dedicated to recording detailed images of small patches of the sky through filters of 17 different colours. The area covered in this image is only about the size of the full Moon, but thousands of galaxies can be identified just within this small region.

The image was taken with an exposure time of almost seven hours, which allowed the camera to capture the light from very faint and distant objects, as well as those that are closer to us. Galaxies with clear and regular structures, such as the spiral specimen viewed edge-on near the upper left corner, are only up to a few billion light-years away. The fainter, fuzzier objects are so far away that it has taken nine or ten billion years for their light to reach us.

The COMBO-17 survey is a powerful tool for studying the distribution of dark matter in galaxies. Dark matter is a mysterious substance that does not emit or absorb light and can only be detected by its gravitational pull on other objects. Some of the closer galaxies pictured act as lenses that distort the light coming from more distant galaxies placed along the same line of sight. By measuring this distortion, an effect known as gravitational lensing, astronomers are able to understand how dark matter is distributed in the objects that act as lenses.

The distortion is weak and, therefore, almost imperceptible to the human eye. However, because surveying the sky with 17 filters allows extremely precise distance measurements, it is possible to determine if two galaxies that appear to lie close to each other are actually at very different distances from the Earth. After identifying the galactic lensing systems, the distortion can be measured by averaging over thousands of galaxies. With more than 4000 galactic lenses identified, this COMBO-17 survey is an ideal method to help astronomers to understand the dark matter better.

This image was taken with three of the 17 filters from the project: B (blue), V (green), and R (red). Data through an additional near-infrared filter was also used.

Links
COMBO-17 at the Max Planck Institute for Astronomy