We’ve just seen concrete confirmation that galaxies could collide and grow in the early universe.
Scientists have finally captured two blazing quasars—galaxies powered by supermassive black holes—merging themselves in the Cosmic Dawn just 900 million years after the Big Bang.
It is the first colliding quasar pair we have found in this epoch. During the intense formation of the universe after the Big Bang, this period should be positively peppered with merging galaxies, but previous searches have only turned up loners.
Astronomers were relieved and delighted to finally find one—a detection that could help reveal more galactic collisions in Cosmic Dawn, now that we have an example to show us what to look for.
“The existence of quasar mergers in the reionization epoch has been expected for a long time,” explains astronomer Yoshiki Matsuoka of Ehime University in Japan. “Now it has been confirmed for the first time.
Quasars are among the brightest objects in the universe. These are the galaxies in which the supermassive black hole feeds at enormous speed: it is surrounded by a huge cloud of dust and gas, from which the black hole directly swallows. This process produces a huge amount of glowing light from the frictional and gravitational forces acting on the cloud, causing it to glow.
Scientists believe that quasars can form when two galaxies merge, a process that results in a higher concentration of material in the galactic center. And we have seen much evidence of past and ongoing mergers in the more recent universe, including galactic centers with two or more supermassive black holes on a slow spiral collision course.
For this reason, and because we have found many quasars in the early universe (in part because they are brighter and therefore easier to see), cosmologists expect a high rate of galaxy mergers during the Cosmic Dawn.
This, in turn, would help us understand the early cosmic period known as the reionization epoch, in which strong light ionized the nebulous neutral hydrogen, causing it to clear up and allowing light to flow freely.
But the search for these fusions has proven to be extremely difficult.
In fact, the discovery was completely accidental. Matsuoka and colleagues were examining data collected with the Subaru telescope when they found something unusual.
“While examining images of quasar candidates, I noticed two similar and extremely red sources next to each other,” says Matsuoka. “The discovery was purely accidental.”
A pair of red balls next to each other can be any number of things. For example, the light of a single object can be split and duplicated by the gravitational warping of space-time between the source and the viewer, causing one object to appear as two or more.
So the scientists made follow-up observations with the Subaru and Gemini North telescopes, as well as with the Atacama Large Millimeter/Submillimeter Array (ALMA).
These observations revealed that not only were the objects real and very distant, they were right next to each other, separated by a gap of only 40,000 light years.
The team also found that much of the light emitted by the galaxies comes from star formation and that they are connected by a gas bridge, revealing that the two are interacting – and in the process of merging.
Each appears to house a supermassive black hole with a mass of about 100 million solar masses. That’s huge for the early universe—even the Milky Way’s central black hole is only 4.3 million solar masses.
It’s a spectacular discovery and one that promises something similar in the future. Meanwhile, scientists are working to analyze the ALMA observations to characterize the dust and gas that surrounds the two galaxies. The results will be published in a separate article.
The research was published in The Astrophysical Journal Letters.