Observations of Galaxy Clusters A399 and A401 with the XMM-Newton Satellite
Irini Sakelliou & Trevor J. Ponman (2004), Monthly Notices of the Royal Astronomical Society
The cosmic web connecting everything in the universe is a vast network of gases—mainly hydrogen—which serves as the essential fuel for star formation. The cosmic web, as seen in the image above, resembles a spider’s web. In the image, stars appear in yellow and later form galaxies. All matter in the universe is distributed along long filaments. Along these threads, stars, galaxies, galaxy clusters, and superclusters are formed.
Looking at the image, one might (not incorrectly) get the impression that some bright yellow, nearly circular structures are galaxy clusters containing many galaxies. These ‘yellow’ galaxy clusters may be on the verge of colliding.
A galaxy cluster emits X-rays. So, the best way to uncover its secrets may be to observe it in X-rays. The European X-ray satellite, XMM-Newton, made 4 observations along the line connecting two clusters—A399 and A401. These clusters are about 1 billion light-years away from us and approximately 40 arcminutes (or 3.2 Mpc) apart. The XMM-Newton satellite has a circular field of view roughly the size of the full moon.
Each cluster leaves its mark in the X-ray image as a prominent dark ‘spot’. The following image was created by combining all four observations.
After analyzing all four sets of data, we produced this image, which we published along with the results of our study in the Monthly Notices of the Royal Astronomical Society. We found that these two galaxy clusters are in the early stages of a collision, before a final merger and the creation of a large supercluster.
The above observations were made using the XMM-Newton X-ray satellite, shown in the next image. It is about 10 meters long and weighs approximately 3 tonnes.
A simulation of a galaxy cluster collision can be seen in the following videos:
Our published article can be found here: https://academic.oup.com/mnras/article/351/4/1439/1135465