Cosmos Editorial
The largest galaxies in the universe may grow up in cosmic oceans of cold gas recycled from previous generations of star formation.
An international team of astronomers analysed radio observations of the young massive Spiderweb galaxy and found it’s sitting in a reservoir of gas that lies outside any galaxy.
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The work, published in Science, provides evidence for computer simulations that suggest that while the heftiest galaxies probably grew by cannibalising smaller galaxies, they started their existence in large reservoirs of cold, soupy gas.
“This is different from what we see in the nearby Universe, where galaxies in clusters grow by cannibalising other galaxies,” said study lead author Bjorn Emonts of the Centre for Astrobiology in Spain. “In this cluster, a giant galaxy is growing by feeding on the soup of cold gas in which it is submerged.”
The Spiderweb Galaxy, which was discovered in the late 1990s, is actually a cluster of protogalaxies more than 10 billion light-years from Earth.
That distance means we’re seeing the cluster as it was when the universe was a mere three billion years old.
It’s thought that right in the centre of such galactic huddles reside the most massive galaxies in the universe – perhaps forming as smaller galaxies are dragged into their gravitational clutches and cannibalised.
Stars (and thus, galaxies) form from molecular hydrogen gas, but it’s very hard to detect. Luckily, carbon monoxide is much easier to “see” with radio telescopes, and can act as a tracer gas for molecular hydrogen.
‘IT APPEARS THAT THIS WHOLE SYSTEM EVENTUALLY WILL COLLAPSE INTO A SINGLE, GIGANTIC GALAXY.’
When the team turned the Australia Telescope Compact Array (ATCA) and the National Science Foundation’s Karl G. Jansky Very Large Array (VLA) towards the Spiderweb galaxy for a total of 98 hours, they found around a third of the carbon monoxide gas was located within the large central galaxy which was in the process of merging with smaller galaxies.
But the rest was in what’s called the intergalactic medium – the regions between the merging galaxies – and appeared to be gas expelled by an earlier generation of star formation because it contains carbon and oxygen, which are blasted from exploding stars called supernovae.
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What’s more, the gas appears to be extremely cold. “It’s shocking how cold this gas must be – about -200 ºC,” said study co-author Matthew Lehnert from the Astrophysical Institute of Paris in France.
“We expected a fiery process – lots of galaxies falling in and heating gas up. Therefore, we expected that all the cold gas would be locked up deep within the galaxies.”
Where the cold gas came from is a puzzle. “The carbon monoxide that we detected is a by-product of previous stars, a form of cosmic recycling, but we cannot say for sure where the gas came from or how it accumulated in the cluster core,” Emonts said.
“To find out we’d have to look even deeper into the universe’s history.”
And astronomers predict big things in the Spiderweb galaxy’s future.
“This is a huge system, with this molecular gas spanning three times the size of our own Milky Way galaxy,” said study co-author Preshanth Jagannathan from the National Radio Astronomy Observatory in the US.
“It appears that this whole system eventually will collapse into a single, gigantic galaxy.”
