Quasars rank among the brightest and most powerful objects in the universe. These are fueled by supermassive black holes at the centers of galaxies that consume surrounding matter, producing so much energy that they can be observed over billions of light-years.
Now, an international team of researchers has identified 31 of the oldest quasars ever discovered, including the two oldest known examples. These unusual objects were already glowing with the light of about 1 trillion suns when the universe was only about 670 million years old. This discovery is astronomy and astrophysicsoffers an unprecedented glimpse into one of the earliest chapters in the history of the universe.
“These objects provide the best clues to understanding how supermassive black holes form,” said co-author Joseph Henawy, a professor of physics at the University of California, Santa Barbara and Leiden University. “These monsters, billions of times the mass of our sun, somehow already existed when the universe was in its infancy. We still don’t quite understand how they grew so large and so quickly.”
Why ancient quasars are so hard to find
Astronomers have been searching for the oldest quasars in the universe for decades. Quasars hold valuable information about how the first galaxies and supermassive black holes formed.
However, they are very difficult to find. Formed less than about 770 million years after the Big Bang, quasars are extremely rare because only a small number of galaxies have grown large enough to host them. Their faint light is easily confused with the light of much closer stars in our galaxy.
Another obstacle is due to the expansion of the universe. As the universe expands over billions of years, the light from these distant quasars changes from ultraviolet to near-infrared. Earth’s atmosphere naturally emits light at these wavelengths, making it very difficult for ground-based telescopes to detect such faint objects.
Astronomers use this effect, known as redshift, to estimate both distance and age. The higher the redshift, the further away and earlier in the history of the universe the object appears. “Redshift 7 takes us to a time when the universe is just 750 million years old, less than 6% of its current age,” Henawi said.
“These two things make it incredibly difficult to find quasars at this distance,” said first author Damin Yang, a doctoral student in Henawy’s group at Leiden University. “There are thousands of stars in our Milky Way and nearby galaxies that look nearly identical in imaging surveys, and their light extends into the infrared at such distances, requiring surveys both wide enough to capture these rare objects and deep enough to detect their faint light.”
These limitations make exploration from the surface nearly impossible. Observing from space provides a clearer view.
Euclid Space Telescope discovers 31 ancient quasars
The European Space Agency launched the Euclid Space Telescope in 2023 to explore the universe during this pivotal time. Operating above Earth’s atmosphere, Euclid will explore vast swaths of the sky at incredible depths, avoiding the infrared glow that limits ground-based observations.
Researchers used data from the Euclidean Wide Survey to identify an unprecedented 31 new quasars from the early Universe. Once the survey is complete, more than a third of the entire sky will be mapped. Some of these newly discovered quasars date from a time when the universe was only about 5% of its current age.
Until now, astronomers have mainly detected only the brightest and rarest ancient quasars, with too few samples to study the entire early quasar population.
“Euclid is a true game changer,” Daming said. “Previously, we were only able to find a handful of very bright ancient quasars, but with Euclid we can search vast areas of the sky much more efficiently and capture much fainter light. This is a unique tool for quasar exploration.”
A window into the first billion years of the universe
Researchers recently took a closer look at the second oldest quasar in the new collection. They discovered that the star resides in a dusty, gas-rich galaxy undergoing an intense burst of star formation, providing new clues about the environment in which early supermassive black holes grew.
These newly discovered quasars originate from a critical period known as the Epoch of Reionization, when the first stars and galaxies transformed the early Universe by ionizing the neutral hydrogen gas that once filled the Universe. This era shaped the subsequent evolution of the universe.
Of the 31 newly discovered quasars, 14 have redshifts of 7 or higher. The two oldest quasars have redshifts of 7.69 and 7.77, making them the oldest quasars ever identified. They are located just over 13 billion light-years away and are thought to have existed during the first 670 million years of the universe. It also surpassed the previous record set by Henawi’s research group in 2021.
“Every time we go back in time, the puzzle becomes more complex: How did the universe produce supermassive black holes so quickly?” Henawi said. “Black holes with masses hundreds of millions of times that of the sun were discovered when the universe was still in its infancy.”
A deeper look into the history of the universe
Through a combination of improved telescopes and more sophisticated exploration techniques, astronomers have steadily moved back in the history of the universe. It took more than a decade to discover approximately the first 10 quasars with redshifts greater than 7. Euclid has already discovered more than that in a single year, more than doubling the known population of these very ancient objects.
Machine learning has also become an integral part of search. Advanced algorithms have enabled researchers to examine tens of millions of astronomical sources and separate the small number of true quasars from the overwhelming number of similar stars and galaxies, Henawy said.
Henawi’s team spent years developing many of the algorithms used to make these discoveries. He also leads the development of PypeIt, the software used by University of California astronomers to process observations collected by the Keck telescope. Keck confirmed two-thirds of the newly discovered quasars, including the three most distant examples, through observational access at the University of California.
Researchers are now aiming to discover the first known quasar with a redshift greater than 8, which would reveal an object that existed within the first 630 million years of the universe.
James Webb and ALMA will study these ancient giants
The discovery of these quasars is just the beginning. The research team has already secured observation time with the James Webb Space Telescope to study many of them in detail. Future observations will measure the black hole’s mass, analyze the chemistry of the surrounding gas, and use its light to track how reionization occurs throughout the young universe.
Meanwhile, the Atacama Large Millimeter Array will study the formation of dust, gas and stars inside the galaxies where these ancient quasars reside, providing a clearer picture of how the oldest giant galaxies evolved.
“The bigger vision is to piece all of this together into a coherent timeline to create the first billion-year quasar chronology,” Henawy said.
Daming Yang, Antoine Baset, and Jean-Charles Cuillandre of the Euclid Consortium contributed to this article.
