Webb Space Telescope deep field

The most ancient galaxies in the universe are coming into view

The James Webb Space Telescope is discovering distant clusters of stars that formed surprisingly early—presenting new mysteries about how the universe evolved.

Using the James Webb Space Telescope, scientists have peered back into the primordial universe and discovered galaxies that existed when the universe was only 300 to 400 million years old. 
Image by NASA, ESA, CSA, M. Zamani (ESA/Webb)

The infant universe was nothing like the cosmos we see today. Early on, it was dark: no stars and no galaxies, just a simmering celestial soup. But hundreds of millions of years after the big bang, the first galaxies formed—exactly how is a mystery—and kicked off an evolutionary sequence that populated the cosmos with galaxies, including our own Milky Way.

Now, astronomers peering into the distant past have found primeval galaxies that may have come into existence earlier and grown faster than predicted.

Light from some of these galaxies, glimpsed by NASA’s James Webb Space Telescope, hurtled through the cosmos for roughly 13.4 billion years before it collided with the instrument’s gold-plated mirrors. Though JWST has been fully operational for only about six months, its observations are already unveiling the early days of galactic history. At least two galaxies JWST has discovered are farther away than anything seen before, and the telescope has spotted additional intriguing candidates awaiting confirmation.

“We are seeing what galaxies looked like at a time when the universe was only 300 to 400 million years old,” Jane Rigby, JWST’s operations project scientist, told a packed auditorium during this year’s winter meeting of the American Astronomical Society in Seattle.

Multiple teams are using JWST to peer into the primordial murk, where ancient galaxies could reveal new insights into the creation story of the universe. That story begins with a bang some 13.7 billion years ago, and the stars and planets and people that populate the universe today are the products of billions and billions of years of cosmic evolution. But their roots are planted in cosmic infancy, hidden behind a veil of dust. JWST, with its exquisitely sensitive infrared eye, was designed to pierce this veil.

Astronomers thrilled by the early successes of JWST are eager to find out just how far back it can see. Already, galaxies that glittered near the beginning of time are presenting new mysteries.

“There are an awful lot of them—too many, too big, too bright, too hot, too mature, and too soon,” NASA’s John Mather, JWST’s senior project scientist, said during the AAS meeting in Seattle. 

Peering through space and time

Telescopes such as JWST are a bit like time machines. When they stare across great distances, they are also staring back in time. The captured light reveals objects as they appeared when they first glimmered, sometimes millions or billions of years ago.

As light travels through an expanding cosmos, it’s stretched to longer, redder wavelengths. Astronomers can measure the extent of that stretch, called redshift, which can be used to calculate an object’s distance. Higher redshifts mean an object is farther away.

Studying cosmic objects at high redshift is one of the JWST’s highest priorities. Perched a million miles from Earth, the telescope sees in the infrared, which makes it an ideal instrument for detecting longer, redder wavelengths of light. Galaxies spotted by JWST are already forcing astronomers to rethink exactly what constitutes high redshift—or “high-z.”

“JWST has absolutely changed our definition of high-z,” writes Guido Roberts-Borsani of the University of California, Los Angeles, in an email. In 2015, he says, the most distant galaxies known had redshift values of 8 or 9. But then the Hubble Space Telescope spotted a galaxy later named GN-z11 around redshift 11 and pushed the first galaxies even further back in time.

“Now JWST has eclipsed that,” Roberts-Borsani says, and the redshift frontier has been moved to values of 12 or 13, equating to about 13.3 or 13.4 billion years ago.

Looking for red

For decades, astronomers have been racing to find the highest redshift galaxies—first with the Hubble and Spitzer space telescopes, and now with JWST. Competition among teams vying to spot the record holder has historically been quite fierce, although that might be changing. When Emma Curtis-Lake, an astrophysicist at the University of Hertfordshire in England, presented new record-breaking galaxies during a December science meeting at the Space Telescope Science Institute in Baltimore, the entire room of astronomers gasped in delight.

“We’ve shifted into a completely new regime—this is the first time we’ve got confirmation of anything further away than Hubble could see, and this is just the beginning,” Curtis-Lake, a member of the JWST Advanced Deep Extragalactic Survey (JADES) collaboration, told National Geographic.

Over the summer, JADES surveyed a well-studied patch of southern sky—a portion of the iconic Hubble Ultra-Deep Field—for primordial galaxies. The collaboration’s galaxy hunters first sifted through the 100,000 galaxies in an image taken by JWST’s Near Infrared Camera. This instrument can measure estimated redshifts based on a galaxy’s light as seen through different filters. Then they aimed another instrument, JWST’s Near Infrared Spectrometer, at the most intriguing targets.

The spectrometer can reveal a galaxy’s precise redshift—and therefore its age and distance—based on characteristic breaks in the spectrum of light coming from the galaxies. Those spectroscopic measurements are far more accurate, says JADES team member Brant Robertson of the University of California, Santa Cruz, which is why astronomers consider them to be confirmation of a galaxy’s redshift.

Curtis-Lake, Robertson, and their colleagues confirmed the distances to four galaxies that populated the primordial cosmos when it was only about 300 or 400 million years old. Two of them, though they are wicked far away, had also been spied by Hubble. The other two are farther away than anything Hubble could see, with redshifts of 12.6 and 13.2. These galaxies are largely made of lighter elements such as hydrogen and helium because they existed before large amounts of heavier elements had time to form.

“They’re sort of like little baby toddlers in a universe that hasn’t really got going yet,” Curtis-Lake says.

A family of early galaxies

Astronomers working on another early galaxy survey, the Cosmic Evolution Early Release Science (CEERS) program, announced confirmations of more galaxies during the AAS meeting, many of which are between redshifts of 8 and 9. The first CEERS mosaic, a composite of 690 individual frames that details a patch of sky near the crook in the Big Dipper’s handle, is the largest JWST galaxy survey image released so far.

“You will spend a long time zooming around these images,” Steve Finkelstein of the University of Texas-Austin warned astronomers in Baltimore. “There’s a lot of fun you can have.”

Although none of the confirmed CEERS galaxies are as far away as the four spotted by JADES, an unconfirmed galaxy that astronomers are still investigating could be whoppingly distant. The candidate appears as a smudge at an estimated redshift of 16, meaning it could be impossibly young and far away. The team also revealed a tomato-shaped galaxy estimated at redshift 12, now known as Maisie’s galaxy after Finkelstein’s daughter.

“It was a really valid candidate, and it was worth writing a paper on my daughter’s birthday, so this is Maisie’s galaxy,” he explained at the Baltimore meeting.

Both these galaxies are awaiting spectroscopic confirmation, and in the meantime, other teams are busy identifying high-redshift candidates among JWST’s other early images. One of those teams, led by Haojing Yan of the University of Missouri, claims to have spotted 87 galaxies spanning redshifts 11 through 20. Those candidates are also awaiting confirmation.

“I’ll bet $20 and a beer—a very tall one—that the success rate ought to be higher than 50 percent,” Yan told reporters during the AAS meeting.

If even a small fraction of those candidate galaxies turn out to be as far away as initially estimated, Yan says, then “our previously favored picture of galaxy formation in the early universe must be revised.”

Testing the laws of nature

At first glance, it appears as though the early universe was more prodigious at cooking up stars and galaxies than scientists anticipated.

“The galaxies we’re finding at those redshifts are more numerous than we expected based on previous observations, and they are also brighter than we expected at those redshifts,” Roberts-Borsani writes. “To fit this ‘new’ picture, galaxies had to start forming earlier and faster than previously thought.”

Roberts-Borsani is a member of the GLASS-JWST collaboration, which is also searching for high-redshift galaxies and studying them to understand cosmic evolution. The GLASS collaboration studied a patch of sky that lies behind a massive cluster of galaxies, and the team has already uncovered a handful of apparently primordial galaxies—more than simulations had predicted. “Something’s a little bit weird over there,” Roberts-Borsani told astronomers in Baltimore.

But, he says, there are ways to explain the apparent overabundance without breaking the currently established laws of the universe. Telescopes like JWST can only image small areas of the sky at one time, so by chance, teams could be studying portions of the sky that are unusually stuffed with galaxies. Another possibility is that these early galaxies are simply brighter than expected, perhaps because star formation worked differently than thought. A third explanation is that estimates based on Hubble observations are incomplete because of Hubble’s limited observing capabilities, and maybe, for still unexplained reasons, the early universe was more efficient at turning the lights on than anticipated.

Answers to these questions may be found in future studies. For now, Finkelstein says, JWST has shown astronomers that the early universe was “more full of stars than we thought!”

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