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Point of impact: astronomers observe collision of galaxy clusters

Galaxy clusters observed upon their first contact, a few hundred million years before the merger. Image by Abell; ROSAT; GMRT/TGSS; ESA/XMM-Newton; NASA/Chandra;  ASTRON/WSRT

An astronomer at the University of Strathclyde have joined an international observation of two galaxy clusters on the verge of colliding, around 1.3 billion light years from Earth.

The study is the first time that this phenomenon, known as a pre-merger shock, has been located by scientists. 

The massive collision between the clusters, named 1E2215 and 1E2216, happened over the course of several billion years and the astronomers were able to observe only a relatively small excerpt – around 40 hours – of the immediate prelude to the event.

However, the discovery is a significant development in a series of programmes aimed at understanding the growth of massive galaxy clusters in the Universe.

Dr Junjie Mao, a Research Associate in Strathclyde’s Department of Physics, participated in the study. He said: "Galaxy clusters are the largest gravitationally-bounded systems in the Universe. Typically, they each contain several hundreds of member galaxies. The space between these galaxies is not empty but filled with hot - several millions of degree in temperature - X-ray emitting gas.

"The full evolution of colliding galaxy clusters is impossible to observe because it lasted for billions of years but we were able to obtain snapshots from different colliding galaxy clusters at different phases. Here, a pair of galaxy clusters featured with a pre-merger shock is witnessed by astronomers.

"This is important because it shows two massive clusters creating an even bigger cluster and behaving in a way we expected from numerical simulations yet never observed in the real world.

"We hope to find many more galaxy clusters with future space telescopes, such as eROSITA and Athena".

Dr Mao was part of the international team which collected observations from the European Space Agency’s (ESA) XMM-Newton observatory and US space agency NASA's Chandra X-ray telescope. The observations were combined with earlier data from the Japanese Aerospace Exploration Agency's now-decommissioned Suzaku satellite and radio data from two ground-based telescopes located in Europe and India, the Low Frequency Radio Array (LOFAR) and Giant Metrewave Radio Telescope (GMRT).

The study was led by Dr Liyi Gu at the RIKEN High Energy Astrophysics Laboratory in Wako, Japan, and the findings have been published in Nature Astronomy.

Dr Mao was funded by a grant from the Science and Technology Facilities Council.