Using a powerful combination of the Subaru Telescope and the James Webb Space Telescope (JWST), astronomers have discovered seven supermassive black hole-powered quasars surrounded by veils of dust that existed when the universe was less than a billion years old.
Supermassive black holes consuming vast amounts of matter and shining as bright quasars while being hidden in thick clouds of dust have long been suspected to exist at an early period in the 13.8 billion-year-old cosmos called “Cosmic Dawn,” but have proved frustratingly elusive.
This is the first detection of hidden but bright quasars in the early universe. It indicates that quasars could actually be twice as common at Cosmic Dawn as previously suspected, researchers said.
“This discovery was only possible with the unique combination of two powerful telescopes,” team leader Yoshiki Matsuoka of Ehime University in Japan said in a statement.
“The Subaru Telescope’s wide and sensitive survey allowed us to spot rare, luminous galaxies, and JWST was able to catch the faint infrared light from the hidden quasars,” Matsuoka added. “This shows how effective the approach of ‘discover with Subaru Telescope, explore with JWST’ can be.”
Quasars at Cosmic Dawn
Supermassive black holes with masses millions or billions of times that of the sun sit at the heart of all galaxies in the modern universe. Not all of these black holes are equal, however. Some, like the supermassive black hole at the heart of the Milky Way, Sagittarius A* (Sgr A*), are quiet because they are not feeding on matter surrounding them.
Others are greedily consuming matter that surrounds them in a flattened, swirling cloud called an accretion disk. The immense gravity of these black holes causes tidal forces in this material that generate intense friction, heating gas and dust in the disk to temperatures as great as millions of degrees. Meanwhile, matter in the disk is channeled to the poles of the supermassive black hole by powerful magnetic fields, from where it is blasted out as near-light-speed jets.
Both of these processes radiate vast amounts of energy across the electromagnetic spectrum that appear to astronomers from great distances as quasars.
Considering how extreme and violent quasars are, it is no surprise that these supermassive black holes are thought to have played a vital role in shaping galaxies, and thus in the evolution of the universe. Yet there is still some mystery surrounding the formation of early supermassive black holes before the universe was a billion years old.
Thus, astronomers have been diligently hunting for quasars that existed during Cosmic Dawn, a period lasting from around 50 million to one billion years after the Big Bang, when the first stars and galaxies are believed to have formed. If there were a large population of supermassive black holes at this time, scientists reason that they must have formed frequently and widely, as a result of the death of the first-generation stars, just as stellar-mass black holes form today.
However, if the number of supermassive black holes was low at Cosmic Dawn, researchers theorize that these cosmic titans formed only in special circumstances, possibly from the direct collapse of vast clouds of gas and dust.
The brightness of quasars should make these supermassive black holes pretty conspicuous even at vast distances, and indeed, the team behind the new research used the Subaru Telescope to discover over 200 quasars. There’s a hitch, however: Quasars are usually spotted by their ultraviolet emissions, but cosmic dust is a very good absorber of this type of radiation.
That means that emissions from heavily shrouded quasars may fail to reach us, which would then mean that the quasars we detect are only a fraction of the feeding supermassive black holes that existed at Cosmic Dawn.
To potentially uncover these hidden quasars, this team turned to a survey conducted with the Hyper Suprime-Cam instrument on the Subaru Telescope (HSC-SSP), looking for very bright galaxies that show signs of high-energy emissions but lack the telltale fingerprints of quasars.
With JWST, they could examine these galaxies in infrared, which left those galaxies as visible light (but was then stretched to longer wavelengths), enabling them to peer through the ultraviolet light-absorbing dust clouds. Using its Near Infrared Spectrograph (NIRSpec), JWST studied 11 of the most luminous galaxies surveyed by the Subaru Telescope between July 2023 and October 2024.
Seven of these galaxies showed clear signs of a quasar, confirming the first dust-obscured luminous quasars discovered at Cosmic Dawn.
Examining the light or “spectra” from these galaxies, the team determined that the quasars are emitting energy equivalent to several trillion suns and are powered by feeding supermassive black holes with masses billions of times that of our star. These characteristics resemble those of unshrouded quasars previously detected at Cosmic Dawn.
The researchers also discovered that the dust surrounding these quasars absorbs around 99.9% of the ultraviolet light they emit and 70% of the visible light they emit. Thus, it is little wonder these cosmic titans have remained so effectively hidden.
The number of quasars over the region of space examined by the team indicate that the population of shrouded quasars is similar to that of unhidden quasars. Thus, the team calculates the population of quasars at Cosmic Dawn to be around double what was previously estimated.
The team now intends to further study these obscured quasars to determine why their environments are so different from those of unshrouded quasars. They also intend to hunt for more shrouded black holes in a wider sample of galaxies that existed in early epochs of the cosmos.
Such work has the potential to reveal the full population of supermassive black holes at Cosmic Dawn, researchers say.
The team’s research was published in the July edition of The Astrophysical Journal.
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