Using the James Webb Space Telescope (JWST), astronomers discovered that when our 13.8-billion-year-old universe was between 4 billion and 6 billion years old, it housed fewer feeding supermassive black holes than previously suspected.
Such supermassive black holes, which can be millions, or even billions, of times as massive as the sun, grow by feasting on matter that surrounds them in the form of flattened disks called accretion disks. These black holes' gravitational influences also heat that material, thereby emitting tremendous amounts of radiation. When a black hole partakes in this extreme process, the whole region (including those radiation jets) is known as an active galactic nucleus, or AGN.
Though supermassive black holes exist in all large galaxies, not all of these gravitationally monstrous objects consume enough matter to reach AGN status. AGNs can radiate so much light they often outshine the combined light of every star in the galaxies they live in.
These findings, delivered by the JWST's Mid-Infrared Instrument (MIRI), offer insight into AGN properties and emphasize challenges associated with discovering these spectacles in the early universe. Supported by the Cosmic Evolution Early Release Science (CEERS) program, the results also hint that our universe may have been more stable than expected during its "teenage" years, which scientists have speculated was its most intense period of star formation.
The team reached their conclusions as they were studying a region of space called the Extended Groth Strip, which sits near the Big Dipper between the Ursa Major and Boötes constellations. The region, which contains an estimated 50,000 galaxies, has been studied extensively — but never with a telescope as powerful as the JWST.
"Our observations were taken in last June and December, and we were aiming to characterize how galaxies looked during the heyday of star formation in the universe," Allison Kirkpatrick, team leader and an assistant professor of astronomy and physics at the University of Kansas, said in a statement. "This is a look back in time of 7 to 10 billion years in the past."
Using MIRI, Kirkpatrick said she and her colleagues looked behind dust in galaxies that existed 10 billion years ago, which can hide cosmic phenomena such as ongoing star formation and growing supermassive black holes.
"So," she added, "I carried out the first survey to search for these lurking, supermassive black holes at the centers of these galaxies.”
Early supermassive black hole galaxies deliver a double suprise
This survey delivered a surprise for Kirkpatrick and her colleagues. They had expected the JWST to find many more AGNs than earlier surveys of the same region, such as one conducted with the Spitzer Space Telescope. But instead, only a smattering of additional feeding supermassive black holes were uncovered.
"The results looked completely different from what I had anticipated, leading to my first major surprise," Kirkpatrick said. "One significant revelation was the scarcity of rapidly growing supermassive black holes. This finding was prompting questions about the whereabouts of these objects."
She suggested this means black holes could be growing at a slower rate than estimated, and added that perhaps black holes' feeding rates were miscalculated by Spitzer because the telescope only allowed astronomers to spot the brightest and most massive galaxies with rapidly growing supermassive black holes.
Those are known to pump out more light than more than supermassive black holes that are feeding at a slower rate, thus making them easier to detect.
The growth of supermassive black holes in the early universe is an important mystery for space scientists to solve because these cosmic titans are believed to influence their surroundings a great deal. They can impact the growth of their host galaxies, for instance, and moderate star formation, thus making them an important element in the overall evolution of the universe.
"The study's findings suggest that these black holes are not growing rapidly, absorbing limited material, and perhaps not significantly impacting their host galaxies," Kirkpatrick continued. "This discovery opens up a whole new perspective on black-hole growth since our current understanding is largely based on the most massive black holes in the biggest galaxies, which have significant effects on their hosts, but the smaller black holes in these galaxies likely do not."
This wasn't the only surprise, however, that these galaxies dropped into the lap of Kirkpatrick and her team. The researchers were also taken aback by an apparent lack of dust in the galaxies they studied.
"By using JWST, we can identify much smaller galaxies than ever before, including those the size of the Milky Way or even smaller, which was previously impossible at these redshifts (cosmic distances)," Kirkpatrick said. "Typically, the most massive galaxies have abundant dust due to their rapid star formation rates."
"I had assumed," she continued, "that lower mass galaxies would also contain substantial amounts of dust, but they did not, defying my expectations and offering another intriguing discovery."
The work could also have implications closer to home regarding the inactive and slowly feeding supermassive black hole, Sagittarius A* (Sgr A*), that sits at the center of the Milky Way.
Basically, our galaxy's supermassive black hole is swallowing so little matter that if it were a human, it would exist on a diet of one single grain of rice every million years. But the team's results could imply that Sgr A* might not have always been such a conscientious eater.
They suggest even the Milky Way may have actually once had an AGN at its heart.
"Our black hole seems quite uneventful, not displaying much activity. One significant question regarding the Milky Way is whether it was ever active or went through an AGN phase," Kirkpatrick said. "If most galaxies, like ours, lack detectable AGN, it could imply that our black hole was never more active in the past.
"Ultimately, this knowledge will help constrain and measure black hole masses, shedding light on the origins of black holes growing, which remain an unanswered question."
The University of Kansas researcher has been granted more time with the JWST to continue her study of the Extended Groth Strip field with MIRI. That means while this current research focuses on just 400 galaxies, future work will center around as many as 5,000 early galaxies.
The team's research has been accepted for publication in the journal Astrophysical Journal, with a post-peer review version available on the paper repository arXiv.