Cosmic Tsunamis Bring Dead Galaxies Back to Life
Galaxies are often found in clusters, which contain what are called "red and dead" members that stopped forming stars in the distant past. Now new research has discovered that giant cosmic tsunamis can bring these dead galaxies back to life.
To explain this phenomenon, the international team of astronomers compares galaxies to that of cities, where thousands of galaxies can be packed together closely, unlike the sparsely populated space around them. Over billions of years, clusters of galaxies can merge with others, like growing cities absorb nearby towns. When the clusters collide, a huge shock wave of energy is released that can drive the birth of a new generation of stars, and the sleeping galaxies get a new lease of life.
While researchers already knew of these giant cosmic tsunamis, until now there was no evidence that the galaxies themselves were affected very much.
Described in the journal Monthly Notices of the Royal Astronomical Society, astronomers Andra Stroe of Leiden Observatory and David Sobral of Leiden and the University of Lisbon came to this revelation after observing the merging galaxy cluster CIZA J2242.8+5301 (nicknamed the "Sausage") located 2.3 billion light-years away.
Using the Isaac Newton and William Herschel Telescopes on La Palma, and the Subaru, CFHT and Keck Telescopes on Hawaii, they found that 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. 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," Stroe said in a statement.
The shock wave, moving at a whopping 9 million kilometers per hour (~6 million mph), actually occurred one billion years ago, when the two original clusters collided. Only now are scientists just seeing this event.
So how exactly does this huge shock wave cause new stars to form? The tsunami causes turbulence in the galactic gas, triggering an avalanche-like collapse that eventually leads to the formation of very dense, cold gas clouds. These are vital for the formation of new stars, and what essentially bring galaxies back from the dead.
But like any good thing, there is a catch.
"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," Sobral added. "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, the researchers say, so they should all have passed through a period of extremely vigorous production of stars.
However, since these shocks only lead to a very brief (in astronomical terms) increase in star formation, the chances of astronomers catching galaxy clusters in the act are slim to none.
The underlying mechanisms behind this phenomenon are still mostly a mystery, so the research team plans to continue studying the Sausage cluster, and determine whether these bursts of star formation need very particular conditions or not. By studying a much bigger sample of galaxies, the team hopes to find out exactly how they happen.
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