Years before she was even sure the James Webb Space Telescope would successfully launch, Christina Eilers started planning a conference for astronomers specializing in the early universe. She knew that if — preferably, when — JWST started making observations, she and her colleagues would have a lot to talk about. Like a time machine, the telescope could see farther away and farther into the past than any previous instrument.
Fortunately for Eilers (and the rest of the astronomical community), her planning was not for naught: JWST launched and deployed without a hitch, then started scrutinizing the early universe in earnest from its perch in space a million miles away.
In mid-June, about 150 astronomers gathered at the Massachusetts Institute of Technology for Eilers’ JWST “First Light” conference. Not quite a year had passed since JWST started sending images back to Earth. And just as Eilers had anticipated, the telescope was already reshaping astronomers’ understanding of the cosmos’s first billion years.
One set of enigmatic objects stood out in the myriad presentations. Some astronomers called them “hidden little monsters.” To others, they were “little red dots.” But whatever their name, the data was clear: When JWST stares at young galaxies — which appear as mere red specks in the darkness — it sees a surprising number with cyclones churning in their centers.
“There seems to be an abundant population of sources we didn’t know about,” said Eilers, an astronomer at MIT, “which we didn’t anticipate finding at all.”
In recent months, a torrent of observations of the cosmic smudges has delighted and confounded astronomers.
“Everybody is talking about these little red dots,” said Xiaohui Fan, a researcher at the University of Arizona who has spent his career searching for distant objects in the early universe.
The most straightforward explanation for the tornado-hearted galaxies is that large black holes weighing millions of suns are whipping the gas clouds into a frenzy. That finding is both expected and perplexing. It is expected because JWST was built, in part, to find the ancient objects. They are the ancestors of billion-sun behemoth black holes that seem to appear in the cosmic record inexplicably early. By studying these precursor black holes, such as three record-setting youngsters discovered this year, scientists hope to learn where the first humongous black holes came from and perhaps identify which of two competing theories better describes their formation: Did they grow extremely rapidly, or were they simply born big? Yet the observations are also perplexing because few astronomers expected JWST to find so many young, hungry black holes — and surveys are turning them up by the dozen. In the process of attempting to solve the former mystery, astronomers have uncovered a throng of bulky black holes that may rewrite established theories of stars, galaxies and more.
“As a theorist, I have to build a universe,” said Marta Volonteri, an astrophysicist specializing in black holes at the Paris Institute of Astrophysics. Volonteri and her colleagues are now contending with the influx of giant black holes in the early cosmos. “If they are [real], they completely change the picture.”
A Cosmic Time Machine
The JWST observations are shaking up astronomy in part because the telescope can detect light reaching Earth from deeper in space than any earlier machine.
“We built this absurdly powerful telescope over 20 years,” said Grant Tremblay, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics. “The whole point of it originally was to look deep into cosmic time.”
One of the mission’s goals is to catch galaxies in the act of forming during the universe’s first billion years (out of its roughly 13.8-billion-year history). The telescope’s initial observations from last summer hinted at a young universe full of strikingly mature galaxies, but the information astronomers could wring from such images was limited. To really understand the early universe, astronomers needed more than just the images; they hungered for the spectra of those galaxies — the data that comes in when the telescope breaks incoming light into specific hues.
Galactic spectra, which JWST started to send back in earnest at the end of last year, are useful for two reasons.
First, they let astronomers nail down the galaxy’s age. The infrared light JWST collects is reddened, or redshifted, meaning that as it traverses the cosmos, its wavelengths are stretched by the expansion of space. The extent of that redshift lets astronomers determine a galaxy’s distance, and therefore when it originally emitted its light. Nearby galaxies have a redshift of almost zero. JWST can handily make out objects beyond a redshift of 5, which corresponds to roughly 1 billion years after the Big Bang. Objects at higher redshifts are significantly older and farther away.
Second, spectra give astronomers a sense of what’s happening in a galaxy. Each hue marks an interaction between photons and specific atoms (or molecules). One color originates from a hydrogen atom flashing as it settles down after a bump; another indicates jostled oxygen atoms, and another nitrogen. A spectrum is a pattern of colors that reveals what a galaxy is made of and what those elements are doing, and JWST is providing that crucial context for galaxies at unprecedented distances.
“We’ve made such a huge leap,” said Aayush Saxena, an astronomer at the University of Oxford. The fact that “we’re talking about chemical composition of redshift 9 galaxies is just absolutely remarkable.”
(Redshift 9 is mind-bogglingly distant, corresponding to a time when the universe was a mere 0.55 billion years old.)
Galactic spectra are also perfect tools for finding a major perturber of atoms: giant black holes that lurk at the hearts of galaxies. Black holes themselves are dark, but when they feed on gas and dust, they rip atoms apart, making them beam out telltale colors. Long before JWST’s launch, astrophysicists hoped the telescope would help them spot those patterns and find enough of the early universe’s biggest and most active black holes to solve the mystery of how they formed.
Too Big, Too Early
The mystery began more than 20 years ago, when a team led by Fan spotted one of the most distant galaxies ever observed — a brilliant quasar, or a galaxy anchored to an active supermassive black hole weighing perhaps billions of suns. It had a redshift of 5, corresponding to around 1.1 billion years after the Big Bang. With further sweeps of the sky, Fan and his colleagues repeatedly broke their own records, pushing the quasar redshift frontier to 6 in 2001 and eventually to 7.6 in 2021 — just 0.7 billion years after the Big Bang.
The problem was that making such gigantic black holes seemed impossible so early in cosmic history.
Like any object, black holes take time to grow and form. And like a 6-foot-tall toddler, Fan’s supersize black holes were too big for their age — the universe wasn’t old enough for them to have accrued billions of suns of heft. To explain those overgrown toddlers, physicists were forced to consider two distasteful options.
The first was that Fan’s galaxies started off filled with standard, roughly stellar-mass black holes of the sort supernovas often leave behind. Those then grew both by merging and by swallowing up surrounding gas and dust. Normally, if a black hole feasts aggressively enough, an outpouring of radiation pushes away its morsels. That stops the feeding frenzy and sets a speed limit for black hole growth that scientists call the Eddington limit. But it’s a soft ceiling: A constant torrent of dust could conceivably overcome the outpouring of radiation. However, it’s hard to imagine sustaining such “super-Eddington” growth for long enough to explain Fan’s beasts — they would have had to bulk up unthinkably fast.
Or perhaps black holes can be born improbably large. Gas clouds in the early universe may have collapsed directly into black holes weighing many thousands of suns — producing objects called heavy seeds. This scenario is hard to stomach too, because such large, lumpy gas clouds should fracture into stars before forming a black hole.
One of JWST’s priorities is to evaluate these two scenarios by peering into the past and catching the fainter ancestors of Fan’s galaxies. These precursors wouldn’t quite be quasars, but galaxies with somewhat smaller black holes on their way to becoming quasars. With JWST, scientists have their best chance of spotting black holes that have barely started to grow — objects that are young enough and small enough for researchers to nail down their birth weight.
That’s one reason a group of astronomers with the Cosmic Evolution Early Release Science Survey, or CEERS, led by Dale Kocevski of Colby College, started working overtime when they first noticed signs of such young black holes popping up in the days following Christmas.
“It’s kind of impressive how many of these there are,” wrote Jeyhan Kartaltepe, an astronomer at the Rochester Institute of Technology, during a discussion on Slack.
“Lots of little hidden monsters,” Kocevski replied.
A Growing Crowd of Monsters
In the CEERS spectra, a few galaxies immediately leapt out as potentially hiding baby black holes — the little monsters. Unlike their more vanilla siblings, these galaxies emitted light that didn’t arrive with just one crisp shade for hydrogen. Instead, the hydrogen line was smeared, or broadened, into a range of hues, indicating that some light waves were squished as orbiting gas clouds accelerated toward JWST (just as an approaching ambulance emits a rising wail as its siren’s soundwaves are compressed) while other waves were stretched as clouds flew away. Kocevski and his colleagues knew that black holes were just about the only object capable of slinging hydrogen around like that.
“The only way to see the broad component of the gas orbiting the black hole is if you’re looking right down the barrel of the galaxy and right into the black hole,” Kocevski said.
By the end of January, the CEERS team had managed to crank out a preprint describing two of the “hidden little monsters,” as they called them. Then the group set out to systematically study a wider swath of the hundreds of galaxies collected by their program to see just how many black holes were out there. But they got scooped by another team, led by Yuichi Harikane of the University of Tokyo, just weeks later. Harikane’s group searched 185 of the most distant CEERS galaxies and found 10 with broad hydrogen lines — the likely work of million-solar-mass central black holes at redshifts between 4 and 7. Then in June, an analysis of two other surveys led by Jorryt Matthee of the Swiss Federal Institute of Technology Zurich identified 20 more “little red dots” with broad hydrogen lines: black holes churning around redshift 5. An analysis posted in early August announced another dozen, a few of which may even be in the process of growing by merging.
“I’ve been waiting for these things for so long,” Volonteri said. “It’s been incredible.”
But few astronomers anticipated the sheer number of galaxies with a big, active black hole. The baby quasars in JWST’s first year of observations are more numerous than scientists had predicted based on the census of adult quasars — between 10 times and 100 times more abundant.
“It’s surprising for an astronomer that we were off by an order of magnitude or even more,” said Eilers, who contributed to the little-red-dots paper.
“It always felt like at high redshift these quasars were just the tip of the iceberg,” said Stéphanie Juneau, an astronomer at the National Science Foundation’s NOIRLab and a co-author of the little-monsters paper. “We might be finding that underneath, this [fainter] population is even bigger than just the regular iceberg.”
These Two Go to Almost 11
But to catch glimpses of the beasts in their infancy, astronomers know they’ll have to push well beyond redshifts of 5 and look deeper into the universe’s first billion years. Recently, several teams have spotted black holes feeding at truly unprecedented distances.
In March, a CEERS analysis led by Rebecca Larson, an astrophysicist at the University of Texas, Austin, discovered a broad hydrogen line in a galaxy at a redshift of 8.7 (0.57 billion years after the Big Bang), setting a new record for most distant active black hole ever discovered.
But Larson’s record fell just a few months later, after astronomers with the JADES (JWST Advanced Deep Extragalactic Survey) collaboration got their hands on the spectrum of GN-z11. At redshift 10.6, GN-z11 had been at the faintest edge of the Hubble Space Telescope’s vision, and scientists were eager to study it with sharper eyes. By February, JWST had spent more than 10 hours observing GN-z11, and researchers could tell right away that the galaxy was an oddball. Its abundance of nitrogen was “completely out of whack,” said Jan Scholtz, a JADES member at the University of Cambridge. Seeing so much nitrogen in a young galaxy was like meeting a 6-year-old with a five o’clock shadow, especially when the nitrogen was compared to the galaxy’s meager stores of oxygen, a simpler atom that stars should assemble first.
The JADES collaboration followed up with another 16 or so JWST observing hours in early May. The additional data sharpened the spectrum, revealing that two visible shades of nitrogen were extremely uneven — one bright and one faint. The pattern, the team said, indicated that GN-z11 was full of dense gas clouds concentrated by a fearsome gravitational force.
“That’s when we realized we were staring right into the accretion disk of the black hole,” Scholtz said. That fortuitous alignment explains why the distant galaxy was bright enough for Hubble to see in the first place.
Extremely young, hungry black holes like GN-z11 are the exact objects astrophysicists hoped would resolve the quandary of how Fan’s quasars came to be. But in a twist, it turns out that not even the superlative GN-z11 is young enough or small enough for researchers to conclusively determine its birth mass.
“We need to start detecting black hole masses at way higher redshift even than 11,” Scholtz said. “I had no idea I would be saying this a year ago, but here we are.”
A Hint of Heaviness
Until then, astronomers are resorting to more subtle tricks for finding and studying newborn black holes, tricks like phoning a friend — or another flagship space telescope — for help.
In early 2022, a team led by Ákos Bogdán, an astronomer at the Harvard-Smithsonian Center for Astrophysics, started periodically pointing NASA’s Chandra X-ray Observatory at a galaxy cluster they knew would be on JWST’s short list. The cluster acts like a lens. It bends the fabric of space-time and magnifies the more distant galaxies behind it. The team wanted to see if any of those background galaxies were spitting out X-rays, a traditional calling card of a voracious black hole.
Over the course of a year, Chandra stared at the cosmic lens for two weeks — one of its longest observation campaigns yet — and collected 19 X-ray photons coming from a galaxy called UHZ1, at a redshift of 10.1. Those 19 high-octane photons most likely came from a growing black hole that existed fewer than half a billion years after the Big Bang, making it by far the most distant X-ray source ever detected.
By combining the JWST and Chandra data, the group learned something strange — and informative. In most modern galaxies, almost all the mass is in the stars, with less than a percent or so in the central black hole. But in UHZ1, mass seems evenly split between the stars and the black hole — which is not the pattern astronomers would have expected for super-Eddington accretion.
A more plausible explanation, the team suggested, is that UHZ1’s central black hole was born when a giant cloud crumpled into a humongous black hole, leaving little gas behind for making stars. These observations “could be consistent with a heavy seed,” said Tremblay, who is a member of the team. It’s “crazy to think about these giant, giant balls of gas that just collapse.”
It’s a Black Hole Universe
Some of the specific findings from the mad spectra scramble over the last few months are bound to shift as the studies go through peer review. But the broad conclusion — that the young universe cranked out a host of giant, active black holes extremely quickly — is likely to survive. After all, Fan’s quasars had to come from somewhere.
“The exact numbers and the details of each object remain uncertain, but it’s very convincing that we’re finding a large population of accreting black holes,” Eilers said. “JWST has revealed them for the first time, and that’s very exciting.”
For black hole specialists, it’s a revelation that has been brewing for years. Recent studies of messy adolescent galaxies in the modern universe hinted that active black holes in young galaxies were being overlooked. And theorists have struggled because their digital models continually produced universes with far more black holes than astronomers were seeing in the real one.
“I always said my theory is wrong and observation is right, so I need to fix my theory,” Volonteri said. Yet maybe the discrepancy wasn’t pointing to a problem with the theory. “Perhaps these little red dots were not being accounted for,” she said.
Now that blazing black holes are turning out to be more than just cosmic cameos in a maturing universe, astrophysicists wonder if recasting the objects in meatier theoretical roles could alleviate some other headaches.
After studying some of JWST’s first images, some astronomers quickly pointed out that certain galaxies seemed impossibly heavy, considering their youth. But in at least some cases, a blindingly bright black hole could be leading researchers to overestimate the heft of the surrounding stars.
Another theory that may need tweaking is the rate at which galaxies churn out stars, which tends to be too high in galaxy simulations. Kocevski speculates that many galaxies go through a hidden-monster phase that sets up a star formation slowdown; they start off cocooned in star-crafting dust, and then their black hole grows powerful enough to scatter the star stuff into the cosmos, slowing star formation. “We might be looking at that scenario in play,” he said.
As astronomers lift the veil of the early universe, academic hunches outnumber concrete answers. For as much as JWST is already changing how astronomers think about active black holes, researchers know that the cosmic vignettes revealed by the telescope this year are but anecdotes compared with what’s to come. Observing campaigns like JADES and CEERS have found dozens of likely black holes staring back at them from slivers of sky roughly one-tenth the size of the full moon. Many more baby black holes await the attention of the telescope and its astronomers.
“All of this progress has been made in the first nine to 12 months,” Saxena said. “Now we have [JWST] for the next nine or 10 years.”
Clarification August 14, 2023:
This article has been revised to clarify that Ákos Bogdán is the leader of the team that used the Chandra X-ray Observatory to study galaxy clusters.