We’ve just seen concrete confirmation that galaxies could collide and grow in the early universe.
Scientists have finally discovered two blazing quasars – galaxies powered by supermassive black holes – that merged into the Cosmic Dawn just 900 million years after the Big Bang.
It is the first colliding quasar pair we have found in this epoch. This period, a time of intense cosmic formation in the aftermath of the Big Bang, should be littered with merging galaxies, but previous searches have turned up only loners.
Astronomers are relieved and happy to have finally found one – a detection that could help reveal more galactic collisions in the Cosmic Dawn, now that we have an example to show us what to look for.
“The existence of merging quasars in the epoch of reionization has long been expected,” explains astronomer Yoshiki Matsuoka of Ehime University in Japan. “It has now been confirmed for the first time.”
Quasars are among the brightest objects in the universe. They are galaxies in which the supermassive black hole feeds at an enormous speed: a gigantic cloud of dust and gas surrounds it, from which the black hole in it definitely slurps. This process produces enormous amounts of bright light due to the forces of friction and gravity acting on the cloud, causing it to shine.
Scientists believe quasars can be formed when two galaxies merge, a process that results in a higher concentration of material in the galactic center. And we’ve seen plenty of evidence of past and ongoing mergers in the more recent universe, including galactic centers with two or more supermassive black holes on a slow, spiraling collision course.
Because of this, and because we have found many quasars in the early Universe (partly because they are brighter and therefore easier to see), cosmologists expect a high number of galaxy mergers during the Cosmic Dawn.
This in turn would help us understand an early universe period known as the Age of Reionization, in which powerful light ionized the misty neutral hydrogen, making it bright and allowing light to flow freely.
But actually finding these mergers has proven extremely difficult.
In fact, the discovery was completely accidental. Matsuoka and colleagues were studying data collected with the Subaru telescope when they noticed something unusual.
“While screening images of quasar candidates, I saw two similar and extremely red sources next to each other,” says Matsuoka. “The discovery was purely coincidental.”
A few red blobs next to each other could be anything. For example, the light from a single object can be split and duplicated by the gravitational distortion of space-time between the source and the viewer, making a single object look like two or more.
Therefore, the researchers conducted follow-up observations using the Subaru Telescope and Gemini North, as well as the Atacama Large Millimeter/Submillimeter Array (ALMA).
These observations showed that the objects were not only real and very far away, but that they were also right next to each other, separated by a gap of only 40,000 light years.
The team also found that a significant portion of the light emitted by the galaxies comes from star formation, and that a bridge of gas connects them, showing that the two are interacting – and in the process with merging.
Both also appear to harbor a supermassive black hole that has about 100 million times the mass of the Sun. That’s huge for the early universe – even the Milky Way’s central black hole has a mass of just 4.3 million solar masses.
It’s a spectacular discovery that promises more of the same in the future. In the meantime, the researchers are working to analyze the ALMA observations to characterize the dust and gas around the two galaxies. The findings will be published in a separate article.
The research was published in The astrophysical diary letters.