Galaxy clusters are like cities where thousands of galaxies can be packed together, at least in comparison to the sparsely populated space around them. Over billions of years, they build up structure in the universe — merging with adjacent clusters, like growing cities absorb nearby towns. When this happens, there is a huge release of energy as the clusters collide. The resulting shock wave travels through the cluster like a tsunami, but until now there was no evidence that the galaxies themselves were affected much.
Stroe and Sobral observed the merging galaxy cluster CIZA J2242.8+5301, nicknamed the “Sausage,” located 2.3 billion light-years away in the direction of the constellation Lacerta. They used the Isaac Newton and William Herschel telescopes on La Palma, and the Subaru, CFHT and Keck telescopes on Hawaii; they found that far from “watching from the back,” the cluster galaxies were transformed by the shock wave, triggering 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,” said Stroe. “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.”
The new work implies that the merger of galaxy clusters has a major impact on the formation of stars. “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,” said Stroe.
“But star formation at this rate leads to a lot of massive, short-lived stars coming into being, which explode as supernovae a few million years later,” said Sobral. “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. 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.”
Every cluster of galaxies in the nearby universe has experienced a series of mergers during its lifetime, so they should all have passed through a period of extremely vigorous production of stars. Given that the shocks will only however lead to a brief — in astronomical terms — increase in star formation, astronomers have to be very lucky to catch the cluster at a time in its evolution when the galaxies are still being “lit up” by the shock.
The next step is to see if the Sausage cluster is unique and that these bursts of star formation need very particular conditions. By studying a much bigger sample of galaxies, the team hope to find out exactly how they happen.