Astronomers have identified the oldest pair of quasars, shining 900 million years after the Big Bang, revealing insights into the epoch of galaxy mergers and reionization of the universe. An international team of astronomers, including members of the Kavli Institute for Universe Physics and Mathematics (Kavli IPMU, WPI), using the Subaru Telescope and Gemini North Telescope on Maunakea, Hawaii, discovered the oldest known pair of quasars. These quasars emit intense radiation as they actively feed supermassive black holes. This significant discovery will help us understand the early evolution of the universe.

About 400 million to 1 billion years after the Big Bang, something, perhaps a combination of sources, released enough radiation to strip electrons from most hydrogen atoms, completely changing the nature of the universe. Quasars are one potential source of radiation that caused this “reionization” of the universe. When matter falls into a supermassive black hole at the center of a galaxy, the matter heats up and emits radiation in a phenomenon known as a quasar.

Rarity of quasar pairs

The collision and merger of two galaxies can feed matter into supermassive black holes, causing quasar activity. So, at a time when intense radiation is consuming the entire universe, one would expect to see a lot of twin quasars, as quasar activity erupts in both colliding galaxies. But during this period of reionization, there was not a single pair of quasars out of the approximately 300 known quasars.

Then, a team of astronomers led by Ehime University Associate Professor Yoshiki Matsuoka, including IPMU’s Kavli Project researcher Masafusa Onue and Professor John Silverman, looked at images taken by the Subaru Telescope and noticed a faint red dot. “While scanning images of quasar candidates, I noticed two similar and very red sources side by side,” Matsuoka says. “The discovery was serendipitous.”

Later observations with the Subaru Telescope and Gemini North Telescope confirmed that the red dots were in fact binary quasars that existed only 900 million years after the Big Bang. This is the oldest quasar pair to merge, and a record for the only pair found during reionization. Later observations also revealed a gas bridge extending between the two galaxies, indicating that the galaxies had in fact merged. This discovery will clarify the role of galaxy mergers and black hole activity at this important stage in the evolution of the universe.