A map of the four galaxy clusters merging in Abell 1758

An article published in “The Astrophysical Journal” reports a study on Abell 1758, a quadruple galaxy cluster formed by two pairs that are in different merger phases. A team of researchers used NASA’s Chandra X-ray Observatory and other telescopes to map this gigantic group of galaxies. Each pair is made up of hundreds of galaxies embedded in enormous amounts of hot gas and invisible dark matter. These accurate observations will help to better assess how long it will take for mergers to occur in the two pairs of clusters, which in the future will in turn merge to form a super cluster that will be one of the most massive objects in the universe.

About 3 billion light years from Earth, Abell 1758 was discovered in 1958 and initially classified as a single galaxy cluster. Only after 40 years the pioneering measurements of X-rays from space thanks to the ROSAT satellite made it possible to understand that actually they were two objects about 2.4 million light-years away from each other that were classified as A1758N (North) and A1758S (South). The two objects are gravitationally connected but show no signs of interaction. Finally, in 2004, observations made with the Chandra X-ray Observatory and ESA’s XMM-Newton space telescope made it possible to understand that the two clusters are also made up of pairs in different phases of merger.

The image (X-ray: NASA/CXC/SAO/G.Schellenberger et al.; Optical:SDSS) shows Abell 1758 putting together X-ray observations of Chandra and other telescopes and optical frequency observations from the Sloan Digitized Sky Survey. In the A1758N pair the two clusters are in an advanced phase of merger and their centers have already passed by each other. Instead, the A1758S pair the two clusters are still getting close to each other. The hot gas emissions of the various clusters are represented in blue and white. In the upper edge of the North pair there’s a shock wave in the gas that allowed the researchers to estimate that the two clusters are moving at a speed between 3 and 5 million km/h with respect to each other.

Other information obtained from the observations concerns the distribution of heavy elements. In the pair A1758N they are found above all between the two centers of the clusters because their merger has influenced that distribution. In the pair A1758S the heavy elements are found above all in the centers of the masses because they have still to begin the real merger therefore the position of the heavy elements has not yet been influenced.

The interactions between the various galaxy clusters are complex and one of the consequences is that even within each cluster the individual galaxies have different velocities. Some galaxies are more influenced by the gravity of the nearby cluster and the attraction increases their velocity.

It will take millions of years before the A1758N and A1758S pairs complete their mergers and it will probably take a few billion years before a single super cluster gets formed. In essence, there’s no hurry to make new observations to get new details on what’s happening in Abell 1758. New instruments will allow to better study an excellent example of galaxy cluster merger.

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