"Dead" galaxies that have long since stopped forming new stars can be brought back to life by a cosmic "tsunami" that can form when clusters of galaxies merge, astronomers are saying.

Most galaxy clusters will include some members that stopped forming stars long ago and are what scientists dub "red and dead."

However, when two or more galaxy clusters collide and merge, that meeting generates a huge shock wave of energy that can result in such a "red and dead" galaxy waking up with a new, albeit temporary, burst of star formation, researchers are reporting in the Monthly Notices of the Royal Astronomical Society.

An international team of astronomers has observed this process occurring in the merging galaxy cluster given the nickname the "Sausage," around 2.3 billion light years distant from Earth.

While the shock wave "tsunami" generated in such merging events has been known for some time, there has been considerable debate as to what effect, if any, it might be having on galaxies in a cluster.

"We assumed that the galaxies would be on the sidelines for this act, but it turns out they have a leading role," said research team member Andra Stroe of Leiden Observatory. "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."

What appears to be happening, the researchers say, is that the shock waves create turbulence in the gas within the cluster, which in turn creates a domino-like collapse resulting in very dense, cold clouds of gas that can then coalesce and form new stars.

The shock wave in the "Sausage," moving at around 6 million miles per hour, was generated a billion years ago in the original collision between two galaxy clusters, but the light from the event is just now reaching the Earth and researchers' telescopes.

All clusters in our neighborhood of the universe will have experienced several mergers during their lifetimes, the scientists explain, and will have had some periods of extremely brisk star production.

However, because those periods would be brief, at least in astronomical terms, catching a cluster in such a "burst" of star formation would be mostly a matter of luck, they say.

"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 David Sobral, who works with the Leiden Observatory and the University of Lisbon.

Those supernova explosions then blow a large proportion of the gas out of the galaxies and with the majority of what's left consumed in star formation, they soon run out of fuel, he explained.

"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."