ESO: An Intergalactic Heavyweight (Abell 901/902)

[bystander]

An Intergalactic Heavyweight (Abell 901/902)
ESO Picture of the Week | 2013 Jan 28

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.

Credit: ESO

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[Ann]

Nice picture. No fewer than five filters have been used for it: U, B, V, R and I. It makes for an interesting starscape, certainly. That very red star appears to be HD 85934, a K5 type giant, obviously very infrared due to its huge, swollen "atmosphere". Its very red color is due to its strong infrared emission, but in visual light HD 85934 is in fact less red than Betelgeuse (which is itself enormously bright in infrared light, too). The bluest star, at upper right, is HD 85690, a star of spectral class class A0, similar to Vega but possibly a little brighter. Its light output peaks in the ultraviolet part of the spectrum, so that it looks very blue here. But it is an intrinsically blue-white star in any case.

Ann