Webb completes his first “deep field” with nine days of observation time. What did it find?

About 13 billion years ago, the stars in the universe’s earliest galaxies sent photons into space. Some of these photons completed their epic journey on the gilded beryllium mirrors of the James Webb Space Telescope in recent months. JWST collected these original photons over several days to create its first “deep field” image.

One of the main goals of the JWST is to study the first galaxies in the universe. The results will help astrophysicists piece together the history of the Universe and the evolution of galaxies. These early galaxies are extremely faint, but JWST was built to find them.

So many questions about galaxies have awaited Webb’s transformative opportunity, and we’re thrilled to play a part in unveiling this story.”

Sandro Tacchella, Astronomer, University of Cambridge, UK

Many things in nature disguise themselves as something else. Only after scientists apply their skills do we approach the truth. Early thinkers thought that everything in the universe revolved around the earth and placed humanity at the center of everything, a confusing misconception that still baffles humanity to this day. Finally we figured it out thanks to Copernicus and those who followed. Natural features on Mars looked like canals built by a Martian civilization, which excited everyone and even fooled some scientists. Eventually, better telescopes revealed the truth. There are endless examples of this.

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Younger galaxies can masquerade as old galaxies, a problem that has plagued our attempts to understand the evolution of galaxies. The early universe was composed almost entirely of two of the lightest and simplest elements, hydrogen and helium. As a result, the stars that make up the earliest galaxies are composed almost entirely of hydrogen and helium. They have “low metallicity” in astronomical jargon.

Confusingly, some younger galaxies also have low metallicity. They should contain much more than just hydrogen and helium because so many stars lived and died before these galaxies formed. And stars forge heavier elements and send them into space as they die, to be picked up by the next generation of stars.

But the JWST is not easily fooled.

Illustration of NASA's James Webb Space Telescope.  The JWST's gold-plated beryllium mirrors and delicate instrumentation make it possible to distinguish young galaxies from older galaxies.  Photo credit: NASA
Illustration of NASA’s James Webb Space Telescope. The JWST’s gold-plated beryllium mirrors and delicate instrumentation make it possible to distinguish young galaxies from older galaxies. Photo credit: NASA

The JWST can dissect the light from these galaxies more precisely than any of its predecessors. It has to be kept ultra-cold to observe infrared light in such detail, which is why it’s far out in space, protected by a massive sunshield. JWST’s near-infrared camera (NIRCam) and near-infrared spectrograph (NIRSpec) were both used in these latest observing efforts, and the teams operating each instrument worked closely to achieve these results.

“It was crucial to prove that these galaxies do indeed inhabit the early Universe. It is very possible that closer galaxies are masquerading as very distant galaxies.” said astronomer Emma Curtis-Lake of the University of Hertfordshire in the UK. “Seeing the spectrum as we had hoped confirmed that these galaxies are at the true edge of our vision, some farther away than Hubble could see! It is a tremendously exciting accomplishment for the mission.”

Two forthcoming papers will present the new results of the JWST deep field observations. One of them is “Spectroscopy of four metal-poor galaxies beyond redshift ten” by Curtis-Lake et al. 2022. The other is “Discovery and Properties of the Earliest Galaxies with Confirmed Distances” by Robertson et al. 2022. None have been peer reviewed to date.

The Hubble Space Telescope introduced us to deep field observations. When astronomers pointed the Hubble at what appeared to be a blank patch of sky in 2003 and 2004 and let it collect photons for more than 11 days, it revealed something extraordinary. What masqueraded as empty space was filled with galaxies. Its ultra-deep-field image found nearly 10,000 galaxies in a tiny patch of dark sky, and astronomers believe about 800 of the faintest and reddest belong to the universe’s primordial galaxies.

The Hubble Ultra Deep Field is seen in ultraviolet, visible, and infrared light.  Image credits: NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI)
The Hubble Ultra Deep Field is seen in ultraviolet, visible, and infrared light. Image credits: NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI)

But we needed a more powerful telescope with better instruments to study them.

NIRCam and NIRSpec were built to find these early galaxies and they are succeeding. The team behind both instruments came together before the telescope was completed and launched to develop JADES, the JWST Advanced Deep Extragalactic Survey. JADES will give astronomers an unprecedented, deep and detailed look at the earliest galaxies in the Universe. “These results are the culmination of why the NIRCam and NIRSpec teams have joined forces to conduct this observing program,” co-author Marcia Rieke, NIRCam Principal Investigator from the University of Arizona at Tucson.

The JWST’s ability to distinguish old galaxies is based on the Lyman Fracture. The Lyman break is related to how neutral gas absorbs light in the star-forming regions of distant galaxies. The further away a galaxy is, the more redshifted its light is. This slight stretch moves the Lyman break to a different position in spectrometric observations. The JWST can detect the Lyman Break with its sharp infrared capabilities.

This image from the JADES survey shows how the JWST can find the Lyman Fracture.  Higher redshift galaxies are the oldest galaxies, and the Lyman break appears at a different place in the spectrometry depending on the age of a galaxy.  In this way, the JWST can distinguish the earliest galaxies in the universe from younger galaxies in a way that Hubble could not.  Image credits: NASA, ESA, CSA, M Zamani (ESA/Webb), Leah Hustak (STScI), Brant Robertson (UC Santa Cruz), S Tacchella (Cambridge), E Curtis-Lake (UOH), S Carniani (Scuola Normale Superiore), JADES collaboration
This image from the JADES survey shows how the JWST can find the Lyman Fracture. Higher redshift galaxies are the oldest galaxies, and the Lyman break appears at a different place in the spectrometry depending on the age of a galaxy. In this way, the JWST can distinguish the earliest galaxies in the universe from younger galaxies in a way that Hubble could not. Image credits: NASA, ESA, CSA, M Zamani (ESA/Webb), Leah Hustak (STScI), Brant Robertson (UC Santa Cruz), S Tacchella (Cambridge), E Curtis-Lake (UOH), S Carniani (Scuola Normale Superiore), JADES collaboration

The JWST captured its deep field in the same region of the sky where Hubble captured its ultra-deep field. Telescopes have been studying this region for about 20 years, creating a complete dataset across the electromagnetic spectrum. The Webb’s observations build on this archive and add the deepest and most light-sensitive observations to date.

JWST’s field is 15 times larger than Hubble’s and is deeper and sharper. The NIRCam image is only the size of a human when viewed from a mile away, but it contains over 100,000 galaxies. Because of the performance of the JWST, astronomers can rest assured that some of them are the earliest galaxies to form in the Universe.

“For the first time we have discovered galaxies just 350 million years after the Big Bang and we can be absolutely sure of their fantastic distances”, co-author Brant Robertson of the University of California Santa Cruz, member of the NIRCam science team. “Finding these early galaxies in such breathtakingly beautiful images is a special experience.”

The team used the telescope’s NIRSpec instrument to study light from 250 of the faintest galaxies in the image over a 28-hour period. The spectra showed precise measurements of each galaxy’s redshift and revealed the properties of the stars and gas in each galaxy.

“These are by far the faintest infrared spectra ever recorded,” said astronomer and co-author Stefano Carniani from the Scuola Normale Superiore in Italy. “They reveal what we hoped to see: a precise measurement of the cut-off wavelength of light due to scattering from intergalactic hydrogen.”

Out of more than 100,000 galaxies in the JWST deep field, the researchers have circled four of them. All four have redshifts greater than 10, with two having redshifts of 13. Redshift tells astronomers how long it took light to reach us (but not how far something is because the universe is expanding). A redshift value of 10 means the light has been traveling for 13.184 billion years. Light from redshift 13 galaxies was among the first rays of light sent out into the universe after the Big Bang.

These are the galaxies astronomers hoped to discover with the JWST, and the telescope is doing just that. These galaxies lit up the cosmic dawn and are crucial to understanding how galaxies form and evolve. “Galaxies forming at these times could be the seeds of much more massive and mature galaxies in the local universe,” Curtis-Lake and co-authors explain in their article.

An artist’s rendering of what the first stars to light up the universe might have looked like at cosmic dawn. Image Credit: NASA/WMAP Science Team

The Cosmic Dawn represents a gap in our understanding of the Universe, and attempts to fill this gap rely on a number of assumptions about gas temperature and other factors. But with its precise instruments, astronomers hope the JWST can fill in the gap with data.

“It is difficult to understand galaxies without understanding the early stages of their evolution. Similar to humans, so much of what happens later depends on the impact of these early generations of stars.” said astronomer and co-author Sandro Tacchella of the University of Cambridge in the UK. “So many questions about galaxies have awaited Webb’s transformative opportunity, and we are thrilled to play a part in unveiling this story.”

In their article, Curtis-Lake and her co-authors acknowledge this milestone in cosmology. “We conclude by emphasizing that this is clearly a milestone for the JWST mission and pushes the spectroscopic frontier to a much earlier epoch of galaxy formation,” they write.

The JWST is just beginning its mission, and finding these Lyman dropouts in ancient galaxies is a critical step. “These JADES observations not only provide clear evidence of the Lyman dropouts up to z = 13.2, but also demonstrate the power of spectroscopy to probe the physics of these galaxies and the IGM,” write Curtis-Lake and her co- authors.

“It’s really just a starting point for the mission.”

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