In a cosmological matchup of “Are they or aren’t they?” the contest is firmly in the former’s favor—10 to one, at last count. The question is one of profound importance: Are the galaxies the James Webb Space Telescope (JWST) is seeing in the early universe really as astonishingly remote as we think they are? So far, the answer is a resounding yes. “The vast majority of these galaxies are being confirmed,” says Steven Finkelstein, an astronomer at the University of Texas at Austin. “It means that everything we saw last summer, that maybe the universe was very proficient at forming stars very early, is going to stand.”
The summer of 2022 saw JWST unleash a torrent of discoveries. After a launch in December 2021 and more than half a year of commissioning, JWST fully switched on in July 2022. Almost immediately thereafter, its unprecedented infrared sensitivity revealed the faint glows of galaxies apparently from the far-distant universe that had formed just hundreds of millions of years after the big bang. Astronomers had expected such landmark results to emerge more gradually. “There was an explosion of data,” Finkelstein says.
Those early results came about so quickly because researchers used a clever shortcut to estimate galactic distances. Astronomers usually pin down cosmic coordinates via precisely measuring redshift, a stretching of a galaxy’s light toward the red end of the electromagnetic spectrum as a result of the universe’s expansion. But this requires the act of assembling and analyzing a galaxy’s spectrum—a time-consuming and subtle process known as spectroscopy. JWST’s firehose of discovery was instead powered by cruder, faster photometry-based techniques that essentially use obvious variations in galaxies’ brightness to estimate their redshift.
Thus, while the photometric results came thick and fast last summer, the spectroscopic results have only just begun trickling out. Already, though, with spectra-based distances in hand from only about a dozen candidates, researchers are finding that most measurements are matching the early photometric results. The latest, published in Nature Astronomy last week, confirm earlier distance estimates for four more galaxies identified by the JWST Advanced Deep Extragalactic Survey (JADES). “We’ve been waiting decades for this,” says Emma Curtis-Lake of the University of Hertfordshire in England, who led the spectroscopic results study. “To be able to do it within the first few months of this telescope was just incredible.”
Of the four, the most distant is one with the somewhat unwieldy name JADES-GS-z13-0. It has a redshift value of 13.2, meaning we are seeing the galaxy as it appeared just 320 million years after the big bang. That high redshift makes JADES-GS-z13-0 the most distant currently known in the universe—a record that JWST seems set to imminently break again but one that highlights why astronomers are so thrilled. We now know for certain we are probing an era of the universe no human has ever laid eyes on before. “It’s astonishing,” says Pieter van Dokkum of Yale University. This galaxy, he evocatively notes, is only slightly older from our perspective than the total time sharks have existed on Earth—some 300 million years. “You go from nothing to these fully formed galaxies in the blink of an eye,” van Dokkum says.
Not all high-redshift candidate galaxies have been so lucky, however, which highlights astronomers’ early caution. In July another survey called the Cosmic Evolution Early Release Science Survey (CEERS), led by Finkelstein, spied a possible galaxy at a redshift of 16.4, just 240 million years after the big bang. Subsequent spectroscopy has shown that deduction was wrong, as revealed in late March in research led by Pablo Arrabal Haro, an astronomer at the National Science Foundation’s NOIRLab. The galaxy is actually a dusty imposter located at a redshift of 4.9, a still impressive but not at all record-breaking 1.2 billion years after the big bang. High levels of star formation are thought to have muddled early photometric analysis. “We can be easily fooled by contamination,” says Callum Donnan of the University of Edinburgh in Scotland, a co-author on the work. “A high-redshift galaxy can be mimicked by a lower-redshift galaxy with different features.”
The good news is that this particular galaxy appears to be a “unique case,” Donnan says. The same study was able to confirm that two other candidate galaxies did not have the same problem. One of these is Maisie’s galaxy, which is seen at a redshift of 11.4, about 400 million years after the big bang, and was named for Finkelstein’s daughter. “She was very excited when I told her it was real,” Finkelstein says.
Now that such galaxies are being confirmed, their scientific implications can be more fully explored. These galaxies are small, many times tinier than the Milky Way. But some appear extremely bright and massive and have high star formation rates similar to that of our galaxy, which forms roughly one new star every year. While the galaxies don’t yet pose problems for leading models of cosmology, they suggest galactic formation began earlier and proceeded faster than expected in the universe, which theorists had previously predicted began churning out galaxies at the ripe age of one billion years after the big bang.
“We’re seeing a rise of massive galaxies faster than we thought previously,” says Fabio Pacucci of the Harvard-Smithsonian Center for Astrophysics. The ages of some of these early galaxies are estimated at just tens of millions of years. This could have implications for large structures of dark matter known as halos that sculpted early galaxies and for the nature of dark matter particles themselves. “One of the big open questions is: What is dark matter?” says Sandro Tacchella of the University of Cambridge. “The first generation of galaxies is a sensitive probe for different dark matter models.”
Some problematic—and potentially model-busting—early-universe candidate galaxies still remain. First among them may be a class of galaxies identified by Ivo Labbé of the Swinburne University of Technology in Australia and his colleagues. The team found galaxies with billions of solar masses, comparable in weight to the Milky Way, from just an estimated 750 million years after the big bang. These galaxies are 10 to 100 times bigger than galaxies previously seen in this era and are packed into structures 30 times smaller than the Milky Way. “They’re small, but they’re massive,” says Labbé, who says JWST is continuing to find similar galaxies essentially anywhere it looks deeply in the sky. For now the galaxies have only been studied by photometry, with spectroscopic analysis planned for July. But the photometric success of other JWST results so far suggests Labbé and his colleagues’ preliminary analysis is correct. “The most extreme galaxies there still seem to pose a problem,” says Michael Boylan-Kolchin of the University of Texas at Austin, who was not involved in the JWST observations discussed in this article. “Some of these systems would have to be forming stars 1,000 times as fast as the Milky Way. The question is: Is that an impossibly high amount of star formation?”
The field continues to change rapidly. An ongoing survey called COSMOS-Webb is expected to deliver many more high-redshift candidates. “Our estimates in the proposal were [to find galaxies] up to a redshift of 10 or so,” says Jeyhan Kartaltepe of the Rochester Institute of Technology, who leads the program. “But those numbers might have been too pessimistic.” Many other astronomers have submitted requests for additional spare time on the telescope to the Space Telescope Science Institute in Maryland, which runs the observatory. More still have submitted proposals for the telescope’s second year of scheduled scientific observations, called Cycle 2, which begins in July.
Some worry the field is moving too fast. While many of JWST’s data, about 80 percent, have a proprietary window of 12 months in which the researchers responsible have exclusive access to their own observations, the rest are open-access. This means that when observations are taken, they are immediately accessible to the public, and anyone can use them. Before Arrabal Haro and his colleagues had published their analysis of the redshift 16.4 galaxy on the preprint server arXiv.org in late March, their open-access work had already been scooped by astronomers on Twitter. “I wanted to do just an extremely simple test,” says Gabriel Brammer of the University of Copenhagen, who posted some of the early results. “The team did a much more detailed analysis. But you can see it instantly if you know where to look.”
Not everyone is happy with such easy access. “You have postdocs who have spent years of their life working on this and making these observations possible,” says Rebecca Larson of U.T. Austin, a co-author of Arrabal Haro’s paper and part of the CEERS team. “Then our data comes out, and it’s public, and people are racing us to the results. We are working on it and also being asked to provide other input for the community. Then other people will come in and put up papers. It’s really frustrating to watch happen.” It is unclear how to resolve the tensions at the moment. “It’d be better if there were some more concrete rules,” says Tom Bakx of Nagoya University in Japan, who was not involved in the research. “Imagine if you have small kids, then it’s simply not possible to spend the entire night calibrating the data. There’s a little bit of a power imbalance. It’s very open competition.”
More positively, the situation appears to have cooled somewhat since the frenetic early weeks of JWST’s operation. Now astronomers are doing what they long dreamed of—gaining their first certain glimpses into an epoch of the universe never studied before. Who knows how much further we will see. “Maybe galaxy formation started already at a redshift of 20,” van Dokkum says, referring to a time a mere 180 million years after the big bang, an epoch scarcely fathomable prior to JWST. If the telescope is showing us anything, however, it’s to expect the unexpected.
Editor’s Note (4/14/23): This article was edited after posting to correct Pablo Arrabal Haro’s last name and the comparison to the total time sharks have existed on Earth.
