ESA Science & Technology - Sequence of merging galaxy clusters

A sequence of galaxy cluster pairs in different merging phases, as observed in X-rays (blue) and radio waves (red) using a variety of different astronomical facilities.

First row: the clusters pair Abell 399 and Abell 401, which are observed several billion years before the merger; second row: the cluster Abell 1758, comprising two smaller, colliding galaxy clusters, seen a couple of billion years before merging; third row: the galaxy cluster pair 1E2216 and 1E2215, which are observed upon their first contact, a few hundred million years before the merger; fourth row: the galaxy cluster CIZA J2242, seen in post merger phase.

Clusters of galaxies are the largest known objects in the cosmos bound by gravity, and can consist of hundreds of galaxies, each containing billions of stars or more. Interspersed between a cluster's galaxies are huge amounts of hot, X-ray emitting gas, and even larger amounts of the invisible dark matter.

They are thought to form gradually, with smaller objects merging into bigger and bigger clusters. While the first touch, the so-called pre-merger phase, lasts for a relatively short period of time – around 100 million years – the entire merging process takes billions of years to complete.

The observation of shock waves propagating along the equatorial plane in the hot gas within the merging pair of galaxy clusters 1E2216 and 1E2215 indicates that the two clusters are seen while coming into contact for the very first time – something that has never been observed before.