Researchers watching the results of collisions between galactic clusters report that the intense energy released is enough to revive ‘dead’ galaxies, even if only temporarily.

Dead galaxies are those that have, over a long period of time, expended all of their energy an stopped forming new stars. Many of the existing stars have expended all of their energy as well leaving the galaxy “red and dead”.

An international team led by Andra Stroe of Leiden Observatory and David Sobral of Leiden and the University of Lisbon have observed the merging galaxy cluster CIZA J2242.8+5301 and watched dead galaxies spring back to life.

According to the researchers it’s a bit like a large city absorbing a small suburb.

“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,” says a statement from the group.

The resulting shockwave acts like a cosmic tsunami, flooding the galactic clusters with energy.

CIZA J2242.8+5301, also known as the ‘Sausage’, is located 2.3 billion light years from Earth located near the constellation of Lacerta in the northern hemisphere.

The researchers used the Isaac Newton and William Herschel Telescopes on La Palma, as well as the Subaru, CFHT and Keck Telescopes in Hawaii to observe the merger. They had initially expected the dead galaxies to be swept along and remain relatively unaffected by the collision, instead they saw a wave of rapid 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,” said 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”, Stroe added.

However, the rapid formation of new stars was not a lasting effect. Far from experience a real rebirth, the new energy was short-lived and left the galaxies even more dead and cold in its wake.

“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. 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,” said Sorbal.

Writing in Monthly Notices of the Royal Astronomical Society the researchers report that because every cluster of galaxies in the nearby universe has been through this process they should all have experienced periods of extremely vigorous star formation at some point.

The researchers hope to find other examples of the phenomenon occurring in other star clusters to confirm what they have observed in the Sausage.