Space hots up when galaxy clusters collide

Galaxy cluster abell 1689.
Galaxy clusters like the one shown above are fascinating astrophysical laboratories.
NASA, ESA, and B. Siana and A. Alavi (University of California, Riverside)

What do you get when a pair of galaxy clusters, both moving at very high speeds, slam into each other?

A whole lot of heat, it turns out, in a finding that has surprised observers.

Astronomers from the University of Colorado at Boulder have been watching two enormous galaxy clusters that are merging to create an even bigger cluster dubbed Abell 115, some 2.4 billion light years away.

In an unexpected outcome, the collision between the two clusters has produced a turbulent region of hot gas, with temperatures reaching 1.7 million degrees Celsius. That’s approximately three times hotter than either of the cluster cores.

Study leader Jack Burns likens the phenomenon to the wake produced by a fast moving motor-boat.

“We did not expect to see such very hot gas between the cluster components,” he says.

“We think the turbulence is like a big spoon stirring up gases, converting the energy of motion from the merging clusters into thermal energy. It is a manifestation of them banging together like two giant pots, something we have not really seen before.”

To estimate the temperature of Abell 115, Burns and his team developed software to create high contrast temperature maps of all the regions in the two colliding clusters, using the X-ray and radio portions of the electromagnetic spectrum.

The results were then fed through NASA’s Ames Research Centre supercomputer.

As well as the unexpected heat produced, the results also show that the newly forming super-cluster is sending radio emissions a long way out into deep space.

“These radio emissions are caused by electrons in the magnetic field of the galaxy cluster traveling at near the speed of light,” says Burns. 

“Clearly something has energised the electrons, which we think is related to the cluster banging process.”

Each of the clusters contains hundreds of galaxies, many the size of the Milky Way. Their collision, says Burns, is one of the “biggest bangs in the universe since the Big Bang”.

The team predicts that eventually the turbulence created by the collision will abate and the new mega-cluster will reach some form of equilibrium.

“We believe Abell 115 will eventually ‘relax’ and become centrally condensed,” says Burns, “which is relatively boring compared to what we are seeing now.”

The findings were presented this week to the 230th Meeting of the American Astronomical Society, held in Austin, Texas.

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