An international collaboration of astronomers has observed a handful of galaxies that might help explain an ongoing mystery in astronomy: How did the biggest galaxies in the universe assemble their mass so quickly?

In a paper published in Nature, the scientists describe the discovery of four ancient galaxies that are forming stars at a furious rate, a hundred times faster than our own Milky Way. The team also believe they have observed the earliest example of a galaxy merger, which happened when the universe was less than 1 billion years old.

“We were looking for something different: for star formation activity in the host galaxies of quasars," lead author Roberto Decarli, from the Max Planck Institute for Astronomy, said in a statement. "But what we found, in four separate cases, were neighboring galaxies that were forming stars at a furious pace, producing a hundred solar masses' worth of new stars per year."

Quasars are galaxies undergoing a special phase. Their supermassive black holes are actively feeding, emitting so much light that they appear as a bright dot in the sky. That’s where the moniker quasi-stellar object comes from. Quasars love messy environments like that around mergers because it gives their supermassive black holes more material to gobble.

Quasars have so far been the only known galaxies with such a high rate of star formation in the early universe (although with one exception). The fact that this frenetic activity is present in companion galaxies might be telling us the important role played by their neighborhood. As is often the case, it’s all about location, location, location.   

"Very likely it is not a coincidence to find these productive galaxies close to bright quasars," added Fabian Walter, who led the observation campaign for these objects using the Atacama Large Millimeter/submillimeter Array (ALMA). "Quasars are thought to form in regions of the universe where the large-scale density of matter is much higher than average. Those same conditions should also be conducive to galaxies forming new stars at a greatly increased rate."

While this discovery is extremely exciting in order to answer questions about the evolution of galaxies, the researchers are cautious. To prove that these are the progenitor objects of the first massive galaxies, future observations need to prove that they are common enough to match with the numbers of big galaxies.

Decarli and his colleagues are now planning a new series of observations with ALMA to hopefully find many more of these early record-breakers.