James Webb Space Telescope

Webb Stares Deeply Into the Universe, Showing How Galaxies Assemble

The James Webb Space Telescope is delivering a deluge of images and data to eager scientists and other hungry-minded people. So far, the telescope has shown us the iconic Pillars of Creation like we’ve never seen them before, the details of very young stars as they grow inside their dense cloaks of gas, and a Deep Field that’s taken over from the Hubble’s ground-breaking Deep Field and Ultra Deep Field images. And it’s only getting started.

True to its main science objectives, the JWST has peered back in time to the Universe’s earliest galaxies looking for clues to how they assemble and evolve.

PEARLS, the Prime Extragalactic Areas for Reionization and Lensing Science, is one of the Webb Telescope’s observing programs. In a new paper in the Astronomical Journal, a team of researchers behind PEARLS explained the program and presented their first findings. The paper is “JWST PEARLS. Prime Extragalactic Areas for Reionization and Lensing Science: Project Overview and First Results.”

The name ‘Prime Extragalactic Areas for Reionization and Lensing Science’ is an unwieldy mouthful of words, but we can break it down to figure out how it’s relevant.

There’s only one way to understand the Universe and what led up to us, and everything else we can observe in the Universe. We have to somehow wind the clock back to long before the Earth, the Sun, our Solar System, or even the Milky Way existed in its present form. Fortunately, the Universe hasn’t expanded so much yet that all the other galaxies have disappeared over the observational horizon.

“The main goal of PEARLS is to study the epoch of galaxy assembly, active galactic nucleus (AGN) growth, and First Light.”

From the paper “JWST PEARLS. Prime Extragalactic Areas for Reionization and Lensing Science: Project Overview and First Results.”

Instead, we can see billions of galaxies in the sky, and some of the light from ancient galaxies and the Universe’s early days is only now reaching us after its 13+ billion-year journey. Scientists know this, and they knew the only way to examine that light in great detail and unearth clues to our origins was to build a powerful, discerning telescope that can look back in time and see the faintest, most red-shifted galaxies. So they built the James Webb Space Telescope with its powerful capabilities to observe in the infrared.

So far, the JWST has lived up to expectations and even exceeded them.

“Webb’s images are truly phenomenal, really beyond my wildest dreams.”

Rogier Windhorst, lead author, JWST PEARLS.

The PEARLS program was born of the need to look back in time to the earliest days of the Universe. The term Extragalactic in PEARLS means the program is looking at fields of galaxies rather than individual galaxies. Reionization refers to the formation of the earliest stars and galaxies energetic enough to reionize the Universe and make it transparent. That signalled the end of the Universe’s Dark Ages and the appearance of the Universe’s first light. Lensing refers to gravitational lensing, which is how the gravity from massive structures like galaxy clusters can act as a lens, amplifying the light from objects behind the cluster. It allows astronomers to study objects at even more extreme distances.

Milestones in the history of the Universe (not to scale). The intergalactic gas was in a neutral state from about 300,000 years after the Big Bang until the light from the first generation of stars and galaxies began to ionize it. That brought an end to the Universe’s Dark Age. The gas was completely ionized after 1 billion years. Image Credit: NAOJ.

Add it all up, and you get PEARLS. “The main goal of PEARLS is to study the epoch of galaxy assembly, active galactic nucleus (AGN) growth, and First Light,” the authors explain in their paper. “PEARLS’ main science goals address JWST’s first two themes: First Light and Reionization, and Assembly of Galaxies, including supermassive black hole (SMBH) growth.”

“The stunning image quality of Webb is truly out of this world,” said Anton Koekemoer, a research astronomer at STScI who assembled the PEARLS images into very large mosaics. “To catch a glimpse of very rare galaxies at the dawn of cosmic time, we need deep imaging over a large area, which this PEARLS field provides.”

PEARLS has captured one of the first medium-deep wide-field images of the cosmos. It features the North Ecliptic Pole region of the sky. The images show how the gravitational lensing from galaxy clusters in the foreground brings more distant objects into view. Some of the distant objects are ancient galaxies interacting with each other. Some of them are Active Galactic Nuclei, extremely luminous regions at the center of galaxies, where black holes superheat material that falls toward them. The AGN images should provide clues to how supermassive black holes (SMBHs) grow so large, an extremely active area of research.

PEARLS has captured one of the first medium-deep wide-field images of the cosmos. This image is only one-quarter the size of the final image and features interacting galaxies with active nuclei. Click the image for a much larger, zoomable version of the image. It’s filled with red-shifted objects that date from the Universe’s early days. Image Credit: SCIENCE: NASA, ESA, CSA, Rolf A. Jansen (ASU), Jake Summers (ASU), Rosalia O’Brien (ASU), Rogier Windhorst (ASU), Aaron Robotham (UWA), Anton M. Koekemoer (STScI), Christopher Willmer (University of Arizona), JWST PEARLS Team
IMAGE PROCESSING: Rolf A. Jansen (ASU), Alyssa Pagan (STScI)

Research Scientist Rolf Jansen is one of the paper’s co-authors. He studies how the earliest galaxies formed and how they evolved into the forms they take today. “I was blown away by the first PEARLS images,” Jansen said. “Little did I know, when I selected this field near the North Ecliptic Pole, that it would yield such a treasure trove of distant galaxies and that we would get direct clues about the processes by which galaxies assemble and grow — I can see streams, tails, shells and halos of stars in their outskirts, the leftovers of their building blocks.”

PEARLS will observe the same regions of the sky four times, making it an important time domain survey.

This image is the PEARLS NIRCam image of the IRAC Dark Field (JWIDF) Epoch-1 at the north Ecliptic pole. The IRAC Dark Field is one of two fields PEARLS will observe, and it’s considered a blank field that’s suited to time domain surveys. PEARLS will observe this field and its other targets up to four times in one year. Image Credit: STScI/Windhorst et al. 2023.

The authors explain how PEARLS will image “… several rich galaxy clusters that boost the signal of faint, high-redshift objects via strong gravitational lensing.” PEARLS observed six galaxy clusters for their gravitational-lensing characteristics. “All of our selected clusters show gravitationally lensed arcs,” the authors explain.

This image is of the El Gordo cluster, a cluster of galaxies chosen for its enormous mass. This image doesn’t show the center of the cluster, but it has a “rich collection of distant lensed source candidates,” according to the authors. STScI/Windhorst et al. 2023.

Research assistant Jake Summers is one of the paper’s co-authors. “The JWST images far exceed what we expected from my simulations prior to the first science observations,” Summers said. “Analyzing these JWST images, I was most surprised by their exquisite resolution.” 

Galaxy clusters are the second-largest type of gravitationally bound structures in the Universe, second only to galaxy filaments. PEARLS will observe two young protoclusters from the Universe’s early age. One of them is the embryonic cluster named TNJ1338-1942. It’s the most distant known proto-cluster and is only about 1.5 billion years old.

This image shows the TNJ1338-1942 protocluster, the most distant known protocluster. It contains a luminous, steep-spectrum radio source. The radio source is an active galactic nucleus, and a future JWST observing program will study it in more detail. The radio source is the irregular orange object in the center. Image Credit: STScI/Windhorst et al. 2023.

PEARLS also imaged the VV 191 pair of galaxies. They’re so faint and red that when the Hubble looked at this region, they were invisible. That’s a testament to the JWST’s capabilities.

VV 191 features an elliptical galaxy (VV 191a) on the left and a spiral galaxy (VV 191b) on the right. The orange arc south of VV 191a is a distant galaxy that’s gravitationally lensed by VV 191a.

The PEARLS NIRCam image of the VV 191 system. VV 191a on the left is gravitationally lensing the red galaxy at 10 o’clock and stretching its light into a curve. Image Credit: STScI/Windhorst et al. 2023.

“For over two decades, I’ve worked with a large international team of scientists to prepare our Webb science program,” lead author Windhorst said. “Webb’s images are truly phenomenal, really beyond my wildest dreams. They allow us to measure the number density of galaxies shining to very faint infrared limits and the total amount of light they produce. This light is much dimmer than the very dark infrared sky measured between those galaxies.” 

Galaxies that were invisible to the Hubble appear in large numbers in JWST images. These first PEARL images show objects as faint as 10 fireflies as far away as the Moon. That’s an incredible achievement. It means the faintest red objects in the images date back to only a few hundred million years after the Big Bang.

The faint light in between stars and galaxies is also an object of interest to astronomers. Scientists cannot abide by unexplained light in the Universe. When astronomers work with images and remove all the light from known sources, like stars and galaxies, a tiny bit of light remains. They call it “ghost light,” and its source is still being investigated. Some astronomers call it the sky’s surface brightness, and it might be related to missing faint galaxies. If they’re there, the powerful JWST should find them.

Third-year astrophysics graduate student Rosalia O’Brien is one of the paper’s co-authors. She designed algorithms to measure faint light between the galaxies and stars that first catch our eye. 

“The diffuse light that I measured in between stars and galaxies has cosmological significance, encoding the history of the universe,” O’Brien said. “I feel fortunate to start my career right now — JWST data is like nothing we have ever seen, and I’m excited about the opportunities and challenges it offers.” 

The fields that PEARL is imaging will likely be monitored throughout JWST’s mission. PEARL will be a time-domain study of the region as it images it four times in one year. But after that, others may study the same region due to its accessibility and desirability as a target.

“I expect that this field will be monitored throughout the JWST mission to reveal objects that move, vary in brightness or briefly flare up, like distant exploding supernovae or accreting gas around black holes in active galaxies,” Jansen said.

This is Centaurus A, the nearest galaxy with an active nucleus. The active nucleus is where a supermassive black hole (SMBH) resides. One of the questions in astrophysics is how SMBHs grow so large, and the JWST should help answer that question by looking at more ancient active nuclei. Image Credit: By ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimetre); NASA/CXC/CfA/R.Kraft et al. (X-ray) – http://www.eso.org/public/images/eso0903a/, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=5821706

“This unique field is designed to be observable with Webb 365 days per year, so its time-domain legacy, area covered, and depth reached can only get better with time,” said lead author Windhorst.

This is just a taste of what the JWST has in store for scientists. In their paper’s conclusion, the authors spell out how the new space telescope will advance our understanding of the early Universe.

The galaxy cluster SMACS 0723 as seen by NIRCam on JWST (Not part of PEARL.) Its gravitational lensing properties (from its mass and from the mass of dark matter) are helping astronomers identify 88 distant galaxies in this field of view for further study. These distant galaxies are the most ancient and are critical to understanding how the Universe takes the shape it does today. JWST images like this also show evidence of dark matter, another question waiting for an answer. Courtesy NASA, ESA, CSA, STScI

“With the enormous new range in both flux and wavelength that the JWST images provide, the community will now have the resources to expand and deepen the study of the morphology, SED (spectral energy distribution), star formation rates, masses, dust content, and extinction at redshifts extending to the epoch of first light, as well as better constrain how much diffuse light may be present in the infrared.”

Young scientists just beginning their careers as the JWST begins its mission aren’t the only fortunate ones. For those of us who grew up on Hubble images, the James Webb is also a source of excitement and discovery. It’ll be fun watching as researchers working with Webb continue to make progress on some long-standing questions.


Evan Gough

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