The largest galaxies in the universe got to their size by cannibalizing other galaxies over many billions of years, but in the early universe, the largest galaxies have long been thought to have formed by devouring intergalactic gas.

The evidence for the different behavior has been limited but now an international team of astronomers has discovered an ancient protocluster feeding from a giant, dense cloud of cold gas. The new object, called the Spiderweb Galaxy, is located 10 billion light-years away and is actually made of several protogalaxies surrounded by 100 billion solar masses' worth of gas.

The research by an international team of astronomers was published in Science and combined the observations of the Australia Telescope Compact Array (ATCA), which studied the extent of the gas, and the National Science Foundation's Karl G. Jansky Very Large Array (VLA), which discovered that two-thirds of the gas is between the protogalaxies.

"This is a huge system, with this molecular gas spanning three times the size of our own Milky Way Galaxy," said co-author Preshanth Jagannathan of the National Radio Astronomy Observatory (NRAO), in a statement. "It appears that this whole system eventually will collapse into a single, gigantic galaxy."

The radio telescopes looked at carbon monoxide present in the cloud and discovered that it had an extremely cold temperature of about -200°C (-328°F). These two facts are crucial for our understanding of galaxy evolution.

Carbon and oxygen weren’t formed in the Big Bang, so they must have been produced by stars in these types of galaxies and then ejected from them. But the ejection mechanism usually warms up the gas to a very high temperature, while this gas is really cold.

The gas falls onto the central protogalaxy, which increases in size and will eventually merge with the other protogalaxies.  

"This is different from what we see in the nearby Universe, where galaxies in clusters grow by cannibalizing other galaxies. In this cluster, a giant galaxy is growing by feeding on the soup of cold gas in which it is submerged," said Bjorn Emonts of the Center for Astrobiology in Spain, who led the team.

This discovery of protogalaxy behavior confirms what simulations have indicated already, that there’s a lot more going on in the early universe than we understand.

This research provides a new tantalizing new look at the lives of galaxies when they began to form.