NASA's James Webb Space Telescope was conceived and designed to explore every phase of cosmic history. Its highest-ranked science goal was to image the first stars and galaxies to form after the Big Bang.

Now, less than a year after the telescope's Christmas Day launch in 2021, an international team of Webb scientists that includes University of Arizona researchers is approaching this goal. The telescope has captured images of the most distant galaxies ever seen. The team has confirmed the galaxies' ages with spectra that place them unquestionably farther than any previously known galaxies. These most distant galaxies are some of the oldest but appear to be the youngest to observers on Earth because it takes so much time for light to travel.

These results, in a pair of papers that have not yet been through the peer review process, reveal four galaxies that date back to just over 300 million years after the Big Bang, when the universe was only 2% of its current age. For perspective, the earliest galaxies confirmed by Hubble Space Telescope date back to 500 million years after the Big Bang, with just one at 400 million years.

The University of Arizona-based team that built the Near Infrared Camera, or NIRCam, onboard Webb partnered with the European Space Agency's Near Infrared Spectrograph, or NIRSpec, team in 2015 to form the JWST Advanced Deep Extragalactic Survey, or JADES, collaboration. The instrument teams proposed JADES as a program that would provide a view of the early universe unprecedented in both depth and detail. NIRCam was to capture images of the galaxies in multiple colors to identify ones that might be very young and distant. NIRSpec would collect spectra for each galaxy – which spread the light like rainbows to search for distinct color patterns – to pinpoint and confirm their distances.

"This is the holy grail of cosmology and why this telescope was conceived," said George Rieke, co-author of the NIRCam paper and a Regents Professor of Astronomy at the UArizona Steward Observatory. He serves as science team lead for another instrument onboard Webb called the Mid-Infrared Instrument, or MIRI.

Astronomers say very distant galaxies have high "redshift." Redshift occurs when light from distant objects is stretched by the expansion of space as it travels from object to observer. The stretched light shifts spectra from its original wavelengths toward longer wavelengths, which appear redder to human eyes. So, the farther away a galaxy is from the observer, the redder it appears. The galaxies captured by Webb are so distant that their redshifts move all their light out of the visible range into the longer wavelengths of the infrared.

"Now that we have the spectra, we're sure what redshift these objects are at," said NIRCam principal investigator and UArizona Regents Professor of Astronomy Marcia Rieke, a co-author on the NIRCam paper. She is also the Dr. Elizabeth Roemer Endowed Chair in Steward Observatory.

"It's gratifying to get what we wanted, and this is just the first glimpse," she said. "We think there are a number of additional very high redshift candidates in our data. We'll get a lot more spectra in the future to do statistical samples of galaxies at various distances. This will help us understand how galaxies change over time. But a lot of galaxy properties are set from the beginning, which makes these very young ones so important. What we're learning from these galaxies is that the gas and dust in the universe left over from the Big Bang is getting organized into galaxies quite rapidly."

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During the first round of JADES observation, NIRCam surveyed in and around an area of the sky called the Hubble Ultra Deep Field. For over 20 years, this small patch of sky has been the target of nearly all large telescopes, building an exceptionally sensitive dataset spanning nearly all wavelengths of light. Now, Webb is providing scientists with the faintest and sharpest images ever in this super-studied region, using its powerful infrared eyes.

NIRCam focused on this region for over 10 days in nine different infrared wavelengths. In total, the observed area is 15 times larger than the deep field itself, but still only about the size of a salad plate viewed across a football field. Yet, the observations include nearly 100,000 galaxies, each caught at some moment in its history, going back billions of years. The faintest and reddest of these galaxies are what the team homed in on, but further investigation was needed to confirm that they were truly the youngest galaxies ever seen.

To test this possibility, JADES then turned to NIRSpec. With a single observation spanning three days, the team collected light from 250 of the faintest galaxies in the region. The NIRSpec rainbow-like spectra separate hundreds of distinct wavelengths in each galaxy. Absorption by gas between us and the distant galaxies imprints a characteristic pattern on the spectra. It is this pattern that provides a precise measurement of the redshift.

In this way, the team identified four of the galaxies to be from an unprecedentedly early time in the universe's history, breaking new ground for the study of galaxies.

"It is hard to understand galaxies without understanding the initial periods of their development," said astronomer and co-author Sandro Tacchella from the University of Cambridge in the United Kingdom. "Much as with humans, so much of what happens later depends on the impact of these early generations of stars. So many questions about galaxies have been waiting for the transformative opportunity of Webb, and we're thrilled to be able to play a part in revealing this story."

JADES will continue in 2023 with a detailed study of another larger field that includes the iconic Hubble Deep Field, and then return to the Ultra Deep Field for another round of deep imaging and spectroscopy.

UArizona and Lockheed Martin's Advanced Technology Center led the design and construction of NIRCam. A consortium of European industrial companies led by Airbus Defence and Space built NIRSpec for the European Space Agency.

Former UArizona professor Brant Robertson – now at the University of California – Santa Cruz – led the NIRCam paper, and Emma Curtis-Lake of the University of Hertfordshire led the NIRSpec paper. Lead co-authors include UArizona assistant research professor Kevin Hainline and graduate student Lily Whitler. Additional co-authors include Harvard University's Daniel Eisenstein, NOIRLab's Christina Williams and University of Texas at Austin's Ryan Endsley – all are former UArizona faculty.

The following researchers are currently from the UArizona NIRCam team: research professor Eiichi Egami; associate research professor Christopher Willmer; associate professor Dan Stark; assistant research professors Stacey Alberts and Irene Shivaei; postdoctoral fellows Zhiyuan Ji, Jianwei Lyu and Michael Topping; and graduate students Jakob Helton, Fengwu Sun, Zuyi Chen and Raphael Hviding.