Galaxy clusters give stars new lease of life

Cosmic 'tsunamis' caused by collisions cause bursts of star formation

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The Abell 2744 cluster is the result of at least four smaller clusters combining into one giant cluster.
Credit: Hubble, ESA, NASA, J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)

By Iain Todd

The shock waves caused when galaxy clusters merge can be enough to jolt dormant galaxies back into life, causing star formation to begin anew.

New research has found that the energy created during the merger of galaxy clusters has a more profound effect on the process of star formation than previously thought.

Galaxy clusters often contain thousands of galaxies packed together. These constantly merge and interact with neighbouring clusters, building up and creating the huge structures of galaxies surrounding massive voids. Unsurprisingly, this merging of clusters creates an enormous release of energy as they collide. This in turn creates a huge shockwave, but until now, it was not thought that this shockwave had any effect on the galaxies themselves.

A research team, led by Andra Stroe of Leiden Observatory and David Sobral of Leiden and the University of Lisbon, have made a discovery that argues evidence to the contrary. Observing the merging galaxy cluster CIZA J2242.8+5301, nicknamed the ‘Sausage,’ the team found that the energy released by the merging had triggered a new wave of star formation.

“We assumed that the galaxies would be on the sidelines for this act, but it turns out they have a leading role. The comatose galaxies in the Sausage cluster are coming back to life, with stars forming at a tremendous rate. When we first saw this in the data, we simply couldn’t believe what it was telling us,” says Stroe.

“Much like a teaspoon stirring a mug of coffee, the shocks lead to turbulence in the galactic gas. These then trigger an avalanche-like collapse, which eventually leads to the formation of very dense, cold gas clouds, which are vital for the formation of new stars.”

Of course, given that every cluster of galaxies will experience numerous mergers during its lifetime, all should theoretically have passed through periods of vigorous star production.

The team plan to follow up their discovery with further analysis of bigger galaxy clusters to determine whether the ‘Sausage’ cluster is unique, and whether these new bursts of star formation require specific conditions in which to occur.

“The explosions drive huge amounts of gas out of the galaxies and with most of the rest consumed in star formation, the galaxies soon run out of fuel,” Sobral adds. “If you wait long enough, the cluster mergers make the galaxies even more red and dead. They slip back into a coma and have little prospect of a second resurrection.”


 

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