After cosmic dark ages, what burned away ubiquitous clouds of gas? NASA telescope finds surprises
During the first billion years after the Big Bang, as the first stars and galaxies switched on and filled the darkness with new light, they also caused another transformation: ionizing the neutral hydrogen gas filling the universe. Until recently, astronomers struggled to explain how it could happen. Ionizing hydrogen requires high-energy ultraviolet (UV) light, and the sparse, fledgling galaxies did not seem up to the task. Now, JWST, NASA’s orbiting infrared observatory, has turned that on its head. It is finding so many bright young galaxies and luminous black holes that there may be an oversupply of UV light.
“We’re beginning to call this an overproduction crisis, because it seems like we just have too many ionizing photons in the universe,” says Pratika Dayal, an astrophysicist at the University of Groningen. That oversupply is intensifying a debate about which UV sources drove this “epoch of reionization” and when it happened. “This story about when reionization occurs … is being rewritten by JWST,” adds astrophysicist Brant Robertson of the University of California (UC), Santa Cruz.
At a much earlier time, 380,000 years after the Big Bang, the roiling primordial soup of protons, electrons, and photons cooled enough for the particles to team up and form neutral hydrogen atoms. The glow of that hot gas is now visible everywhere in the sky as the cosmic microwave background (CMB). Tens of millions of years of cosmic “dark ages” followed, until the denser clouds of hydrogen coalesced into the first stars.
In today’s universe, hydrogen between galaxies is almost entirely ionized. A clue to when it happened comes from the CMB photons themselves, which get scattered by free electrons from the ionized hydrogen. Patterns in the CMB indicate the midpoint of reionization occurred about 700 million years after the Big Bang.
Another clue comes from quasars, which are supermassive black holes at the centers of distant galaxies that cause gas to glow brightly as it swirls into the gravitational sink hole. Specific wavelengths of quasar light are absorbed by intervening clouds of neutral hydrogen. By the time the universe was about 1 billion years old, those absorption lines fade out, indicating the disappearance of the last remaining neutral hydrogen.
It wasn’t until about 2009, when NASA installed the latest instruments on the Hubble Space Telescope, that astronomers began to get their first glimpses into the epoch of reionization, finding a handful of galaxies dating back to within the first billion years after the Big Bang. But Hubble could only see the brightest galaxies, and they were too few and far between to ionize the whole universe. Was there a population of smaller galaxies blasting out UV light but still unseen?
JWST’s answer to that question is a resounding yes. Since it began observing in 2022, it has found more than 1000 candidate galaxies within the first 1 billion years. In May, Robertson’s team reported confirming one galaxy that was shining less than 300 million years after the Big Bang. “The universe has found a mechanism to grow galaxies, both in luminosity and in size, maybe more rapidly than had been anticipated before,” he says.
The first stars in those galaxies may have been giants, without the heavier elements that radiate heat and limit the size of today’s stars. Such hot, bright giants would be prolific producers of UV light. Evidence for that came in February, when a team led by Hakim Atek of the Paris Astrophysics Institute reported the results of a JWST survey of eight ultrafaint galaxies from the first billion years whose light had been boosted by the “gravitational lens” of a cluster of galaxies closer to Earth. That allowed JWST to split the light into spectra, revealing that the galaxies were putting out four times as much UV as similar galaxies in later times.
A remaining unknown is how much of that UV would escape the galaxies. In more recent galaxies dust and hydrogen gas absorb much of it, and only a fraction escapes. But if the eight galaxies are representative of the wealth of galaxies JWST is finding, “it’s enough to reionize the entire universe,” Atek says—even if only a fraction is escaping.
That early brilliance poses a timing problem, says Julian Muñoz of the University of Texas at Austin. Based on the wealth of early galaxies discovered by JWST and estimates of their UV output and escape fraction, reionization should have started earlier and finished faster than the CMB data and quasar data suggest, he and his colleagues argue in a paper recently accepted for publication. “There’s way too many photons,” Muñoz says. “Something has to give. One of our assumptions has to break.”
Not everyone is convinced that the problem is so severe. “Crisis is, I feel, a little bit of a strong statement because there are some outs,” Robertson says, such as the large uncertainties over UV production and escape fraction. “Escape fraction is the hardest thing to measure,” Atek says.
Some argue that the dominant driver of reionization was not stars but active galactic nuclei (AGNs)—the black hole–powered glowing galactic centers, of which quasars are the brightest. AGNs are much more efficient producers of UV light than stars and, in later times at least, allow much more of it to escape. But they were long disregarded as engines of reionization because they are widely scattered and were thought to form too late to play a major role.
JWST is confounding that view, too. It has found up to 100 times more AGNs about 1 billion years after the Big Bang than suggested by x-ray telescopes, says Piero Madau of UC Santa Cruz. “It was a total surprise.” AGNs are known to have driven the reionization of the universe’s neutral helium—a parallel process that finished later. So why not hydrogen, too, Madau asks. “I think it is interesting to use Occam’s razor and test a scenario where AGNs are the sole drivers of the cosmic hydrogen/helium reionization process.”
Dayal believes the truth lies somewhere in between. In January, she and her colleagues reported the results of a galaxy formation model tuned to some of the new JWST data. They found that three-quarters of the ionizing photons came from small galaxies and one-quarter from AGNs. “I think low-mass galaxies, in principle, are the key drivers of reionization,” she says, but “you could have a contribution from black holes towards the end.”
Far from answering the questions surrounding reionization, JWST seems to have blown the issue open, says Rohan Naidu of the Massachusetts Institute of Technology. “It seems like suddenly everything is back on the table,” he says.
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