Webb Telescope spots the two most distant galaxies ever seen at cosmic dawn

a collection of galaxies, with a pullout of one blurry reddish galaxy

This infrared image from NASA’s James Webb Space Telescope was taken by the onboard Near-Infrared Camera for the JWST Advanced Deep Extragalactic Survey, or JADES, program. The NIRCam data was used to determine which galaxies to study further with spectroscopic observations. One such galaxy, JADES-GS-z14-0 (shown in the pullout), was determined to be at a redshift of 14.3, making it the current record-holder for most distant known galaxy. This corresponds to a time less than 300 million years after the big bang.

NASA, ESA, CSA, STScI, Brant Robertson (UC Santa Cruz), Ben Johnson (CfA), Sandro Tacchella (Cambridge), Marcia Rieke (University of Arizona), Daniel Eisenstein (CfA), Phill Cargile (CfA)

When Kevin Hainline saw the data on his screen, he jumped up from his desk, startled by what he saw.  

In January 2023, Hainline – an associate research professor in the University of Arizona Department of Astronomy and Steward Observatory – was the first person to ever lay eyes on the galaxies JADES GS-z14-0 and the slightly less distant GS-z14-1. At the time of their discovery, these galaxies were merely two candidates among the hundreds Hainline had discovered, each one requiring careful follow-up observation to help confirm their extreme distances.

Months later, standing over his desk, he had data confirming that the international team of astronomers working on the James Webb Space Telescope Advanced Deep Extragalactic Survey, or JADES, had actually found the two most distant galaxies ever seen by humans. 

The data that had gotten Hainline and members of the JADES collaboration so excited had traveled as light for quite a long time, emitted from these galaxies when the universe was very young. They were seeing GS-z14-0 when the universe had only existed for 290 million years – only 2% of its age, from an era astronomers call the cosmic dawn. The other galaxy, GS-z14-1 appears as it was roughly 300 million years after the universe began with the Big Bang. 

While the first few years of NASA's James Webb Space Telescope observations had uncovered a wealth of these ultra-distant galaxies, the JADES scientists were stunned by how GS-z14-0 was bright enough to be seen with the telescope's Mid-Infrared Instrument, or MIRI, Hainline said.

They were also surprised that it appears to be more mature than astronomers would expect, bursting with many massive stars and displaying the signatures of unexpected elements that shouldn't have yet evolved in a galaxy so early in the universe's history, according to current understandings of how and when the first galaxies formed.

The JADES team published its discovery and interpretation of the galaxies in a trio of papers on arXiv, a free online archive of research papers.

The first paper, led by Stefano Carniani from the Scuola Normale Superiore in Pisa, Italy, confirms the distance of GS-z14-0. Hainline is the paper's second author and pinpointed the galaxy from images taken with the Near Infrared Camera, or NIRCam, instrument onboard the James Webb Space Telescope, or JWST. Jakob Helton, a UArizona graduate student, led the second paper, which reports the properties of the galaxy using data collected by JWST's Mid-Infrared Instrument, or MIRI. Brant Robertson, a University of California Santa Cruz astronomer, led a third paper addressing the conundrum of how such a massive galaxy could have formed so early.

The first two papers, which have not yet been formally peer-reviewed, have been submitted to two different high-impact journals. The third has been accepted for publication in the Astrophysical Journal.

Beyond belief

One of the most surprising aspects of GS-z14-0 was how "stupendously bright" the galaxy was, Hainline said. It was initially detected in deep images taken with JWST's very sensitive NIRCam. Follow-up observations, which Helton analyzed using data from MIRI, revealed the galaxy glowing even brighter at longer wavelengths of light. The JADES scientists were puzzled. While the galaxy had all the hallmarks of being very distant, it was seemingly too bright to be at that distance.

"All the distant galaxies that we've found thus far were pretty faint," Hainline said. "They all look like a little smudge."

This includes the previous record holder, GS-z13-0, discovered by the JADES team in 2022 at redshift 13.2, seen 325 million years after the Big Bang. Astronomers use redshift to explain how distant a galaxy is from Earth. As light travels from these distant galaxies, it is stretched to longer wavelengths by the expansion of the universe. The most distant galaxies are moving fastest away from us, and their light has undergone the most stretching, or redshift. 

For comparison, GS-z14-0 was found at redshift 14.3, and it's almost five times brighter than the old record holder. GS-z14-1, at a redshift of 13.9, is similar to GS-z13-0, and is the type of galaxy Hainline expected to find at such an extreme distance.

One other aspect of GS-z14-0 that made Hainline pause was that another galaxy partially obstructed the view of the distant source, confusing scientists' understanding of it. 

"It looked like it might be part of the same object," Hainline said. "Either GS-z14-0 was a strange, dusty object relatively close to us, like its neighbor, or it was an unbelievable discovery, and we were the victim of an extraordinary set of coincidences."

But once the team was able to tease out the details in the data, the evidence for its great distance was strong.

"The fact that this galaxy was seen at long wavelengths with MIRI by Jake (Helton), and the fact that it was even brighter in the MIRI image than with NIRCam, was one of the crucial pieces that led us to thinking the source could be so far away," Hainline said. MIRI sees longer wavelengths than NIRCam, and more distant galaxies are stretched to longer wavelengths.

To confirm that these galaxies are as distant to us as they appear, astronomers look at a galaxy's spectrum, which is made by using a prism to spread the light from the source, and examine the intensity of different wavelengths.

For high-redshift galaxies, astronomers focus on the short wavelength ultraviolet light that a galaxy produces from very young stars. There's a cliff in the spectra of galaxies that are forming stars, called the Lyman break, which is caused by hydrogen gas from around the galaxy absorbing ultraviolet light. This cliff is a very clear marker that astronomers can use to calculate how much a galaxy's light has redshifted and verify that it's truly at the predicted distance.

If GS-z14-0 wasn't as distant as they hoped, a spectrum of the source would show some light at shorter wavelengths than the potential Lyman break.

The JADES team observed GS-z14-0 for almost 10 hours in January 2024, taking an ultra-deep spectrum using the NIRSpec instrument, Hainline said. They were stunned when the spectrum showed a strong Lyman break. 

"The fact that we see nothing on the other side of this break is unambiguous evidence that this galaxy is very distant," Hainline said. "We were being conservative with this object because it's really tricky, just from images of a galaxy alone, to estimate its redshift, and there are many ways in which a galaxy can look like it's ultra distant, but in reality, is a close-by interloper. Given all that we knew about the source, I was worried that this was too good to be true. The universe can be very mischievous."

A plot depicting the spectrums of the three most distant galaxies

Scientists used NASA’s James Webb Space Telescope's Near-Infrared Spectrograph to obtain a spectrum of the three most distant galaxies GS-z14-0, GS-z14-1 and GS-z13-0 to accurately measure its redshift and therefore determine its age. The farther a spectrum is shifted to the right, the more distant it is. The redshift can be determined from the location of a critical wavelength known as the Lyman break. GS-z14-0 is the most distant galaxy ever discovered and dates back to less than 300 million years after the big bang.

Kevin Hainline

The reason the galaxy appeared so bright at the longer wavelengths observed by MIRI is because of an abundance of nebular gas between the stars. Like a neon sign, radiation from massive stars excites and ionizes the nebular gas, causing it to glow brightly, Helton said. This process produces distinct signatures in the galaxy's light, which indicates the presence of hydrogen and, surprisingly, oxygen.

"At the predicted distance, glowing hydrogen and oxygen gas in the galaxy would naturally explain the bright MIRI observation," Helton said.

This is significant because oxygen is only created in the bellies of stars or in their explosive deaths. Multiple generations of stars must have lived and died for GS-z14-0 to look the way it does.

It's thought that the universe only contained the conditions for galaxies to evolve around 200 million years after the Big Bang, corresponding to a redshift of 20, said co-author and MIRI lead scientist George Rieke, a UArizona Regents Professor of astronomy. This means that GS-z14-0 had less than 100 million years to produce oxygen.

The presence of so many massive stars within GS-z14-0 hints at another problem for astronomers to untangle.

"There's this concept of dark halos. They're clumps of dark matter concentrated where galaxies form," Rieke said. "Simulations show that they grow as the universe ages. And the problem with this galaxy is it's pushing against what we think is the maximum mass for a dark halo at that time."

More discoveries await

Hainline compared finding oxygen in this galaxy from the very early universe to finding a smartphone in the archaeological ruins of ancient Rome. 

"It raises the question, if you find the smartphone of GS-z14-0, where are the calculators? Where are the TVs? Where are all the things that are slightly less evolved and less impressive?" Helton said. "Also, if we found something this bright in such a small area of the sky, it naturally makes us think that there are many more galaxies out there that are similar but as of yet undiscovered."

GS-z14-0 was discovered in an area of the sky that appears to be as large as a grain of sand held at arm's length, Hainline said, so galaxies like it could potentially be common.

When Hainline found GS-z14-0 last year, he immediately messaged Helton using the JADES Collaboration Slack online messaging channel, where they share interesting and mysterious galaxies they find as they look through the data.

"The JADES Slack is a living document of human beings grappling with the unknown," Hainline said. "I love the fact that I've got experts at my fingertips to give feedback. The papers we submit have gone through our own internal review. Jake and the other co-authors have had to revise hundreds of comments about these papers, which is a good thing for the paper." 

MIRI's magic

The JADES team never thought MIRI was capable of a discovery such as this.

"For many years, MIRI was the first instrument to go in the event of budget cuts," Rieke said, "and that was in part because the people were really focused on the very high redshift galaxies" that NIRCam was expected to find. 

"They didn't think MIRI could contribute anything to that," Rieke said. "But it's more sensitive than we expected, so these discoveries are really gratifying. The other part of the story is that nobody dreamed that there would be galaxies this bright at this high redshift."

Helton said he wasn't expecting to find anything like this. When Hainline brought him the first evidence that GS-z14-0 was as distant as he thought it was, Helton was using MIRI to look for galaxies at redshift 8, corresponding to roughly 630 million years after the Big Bang. He expected to find a few at that redshift and ended up finding more than 20.

"It was already outperforming expectations," Helton said.

Marcia Rieke – UArizona Regents Professor of astronomy, NIRCam principal investigator and George Rieke's wife – didn't anticipate a discovery like this, but she was excited about MIRI being a part of the JWST payload.

"I viewed MIRI as the instrument onboard JWST that offered us the most discovery space," she said. "There's never been an instrument in space like MIRI."

"The 20,000 people that helped build JWST did an incredibly good job to have this just working exactly the way we dreamed it would work, and even better than expected," George Rieke said.

Going forward, the JADES team will continue "marching our way back in time," Hainline said, "if only because of the mysteries we have been continually presented with, like GS-z14-0."

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