‘Red and dead’ galaxies that stopped forming stars in the distant past can sometimes come back to life, find two studies published in the journal Monthly Notices of the Royal Astronomical Society.
Surface brightness map of the merging cluster CIZA J2242.8+5310. Scale bar – 1.63 million light-years. Image credit: G. A. Ogrean et al., arXiv: 1403.5273.
According to structural formation hierarchy of the Universe, stars form galaxies and galaxies form clusters.
Gigantic galaxy clusters can merge, forming the largest gravitationally bound structures in the Universe. When this happens, there is a massive 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 very much.
Dr David Sobral of Leiden Observatory and the University of Lisbon, Andra Stroe of Leiden Observatory, and their colleagues, used the Isaac Newton and William Herschel Telescopes on La Palma, and the Subaru, CFHT and Keck Telescopes on Hawaii, to observe the merging galaxy cluster CIZA J2242.8+5301, also known as the Sausage. The cluster lies in the constellation Lacerta, approximately 2.3 billion light-years away.
The astronomers found that far from ‘watching from the back’ the Sausage 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.
The results of the two studies imply that the merger of galaxy clusters has a major impact on the star formation.
“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 said.
A radio image highlighting the shock wave – seen here as the bright arc running from bottom left to top right – in the cluster CIZA J2242.8+5310. The shock wave was generated 1 billion years ago, when the two original clusters collided, and is moving at a very high speed of 9 million km per hour. Scale bar – 1 million light-years. Image credit: Andra 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,” Dr 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.”
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