James Webb Telescope Discovers Tiny Galaxies Fueling Cosmic Evolution

Updated on: June 11, 2025 | By: Jameswebb Discovery Editorial Team

NASA’s James Webb Space Telescope (JWST) has uncovered a hidden population of small, star-forming galaxies that played a pivotal role in transforming the early universe. These diminutive cosmic powerhouses, observed in the galaxy cluster Abell 2744, were instrumental in a process known as reionization, which cleared the cosmic fog of neutral hydrogen and shaped the universe we know today. Announced on June 11, 2025, this discovery highlights the telescope’s unmatched ability to peer into the distant past, revealing the unsung heroes of cosmic evolution. This article delves into the science behind these findings, the unique properties of these galaxies, and their profound implications for our understanding of the universe’s infancy.

A Cosmic Makeover: The Role of Tiny Galaxies

The early universe, just a few hundred million years after the Big Bang, was a vastly different place—a dense, opaque fog of neutral hydrogen gas shrouded the cosmos, blocking light from emerging stars and galaxies. Over time, this fog was cleared through a process called reionization, where ultraviolet light ionized the hydrogen, making the universe transparent. Astronomers have long debated which cosmic objects drove this transformation: massive galaxies, supermassive black holes, or smaller, less conspicuous players.

A groundbreaking study, presented at the 246th meeting of the American Astronomical Society in Anchorage, Alaska, on June 11, 2025, points to an unexpected answer. Using data from JWST’s Near-Infrared Camera (NIRCam) and Near-Infrared Spectrograph (NIRSpec), researchers led by Isak Wold at Catholic University of America and NASA’s Goddard Space Flight Center identified 83 small, low-mass galaxies that were prolific star factories 800 million years after the Big Bang. These galaxies, observed through the gravitational lens of the massive galaxy cluster Abell 2744, were key players in reionization, producing vast amounts of ultraviolet light that ionized the surrounding hydrogen gas.

“When it comes to producing ultraviolet light, these small galaxies punch well above their weight,” Wold said. “Our analysis is 10 times more sensitive than previous studies, showing that these galaxies existed in sufficient numbers and packed enough ultraviolet power to drive this cosmic renovation.”

Abell 2744: A Cosmic Lens into the Past

Located 4 billion light-years away in the constellation Sculptor, Abell 2744, nicknamed Pandora’s Cluster, is a colossal assembly of galaxies whose immense gravity warps spacetime, creating a natural magnifying glass known as a gravitational lens. This effect amplifies the light from distant, faint objects, allowing JWST to peer deeper into the universe’s history than ever before. The discovery of these starburst galaxies was part of the UNCOVER (Ultradeep NIRSpec and NIRCam Observations before the Epoch of Reionization) program, led by Rachel Bezanson at the University of Pittsburgh.

By sifting through NIRCam images, Wold and his Goddard colleagues, Sangeeta Malhotra and James Rhoads, identified 83 young galaxies bursting with star formation at a cosmic epoch known as redshift 7, when the universe was about 800 million years old—roughly 6% of its current age of 13.8 billion years. The team selected 20 of these galaxies for detailed study using NIRSpec, focusing on their light signatures to confirm their role in reionization.

The gravitational lensing of Abell 2744 was critical to this discovery. “The cluster’s mass acts as a natural magnifying glass, allowing us to see these tiny galaxies as they were when the universe was young,” Malhotra explained. This magnification, ranging from 3 to 13 times for the studied galaxies, revealed details that would otherwise be too faint for even JWST’s powerful instruments.

Starburst Galaxies: Small but Mighty

These newly discovered galaxies are remarkably small, with stellar masses ranging from 2 million to 160 million times that of our Sun—compared to the Milky Way’s estimated 1 trillion solar masses. Despite their size, they are prolific star factories, undergoing intense bursts of star formation known as starbursts. These episodes produce copious amounts of ultraviolet light, which is critical for ionizing neutral hydrogen.

The team targeted galaxies emitting strong signals of doubly ionized oxygen, a hallmark of high-energy star-forming processes. Originally emitted as green visible light in the early universe, this glow was stretched into the infrared by the universe’s expansion, making it detectable by JWST’s NIRCam and NIRSpec. “Low-mass galaxies gather less neutral hydrogen gas around them, which makes it easier for ionizing ultraviolet light to escape,” Rhoads said. “Starburst episodes also carve channels into a galaxy’s interstellar matter, helping this light break out.”

One standout galaxy, dubbed 41028, has a stellar mass of just 2 million Suns—comparable to the largest star clusters in our Milky Way. Yet, its vigorous star formation made it a significant contributor to reionization. If these galaxies release about 25% of their ultraviolet light into surrounding space, as similar present-day galaxies (known as green peas) do, they could account for all the ultraviolet light needed to ionize the universe’s hydrogen gas. This finding suggests that small galaxies, rather than their larger counterparts or supermassive black holes, were the primary drivers of this cosmic transformation.

The Science of Reionization

Reionization, occurring roughly between 100 million and 1 billion years after the Big Bang, was a pivotal epoch in cosmic history. It marked the transition from the Cosmic Dark Ages, when neutral hydrogen blocked most light, to a transparent universe where galaxies and stars could shine freely. JWST was designed to study this period, using its infrared capabilities to observe light from the early universe that has been redshifted by cosmic expansion.

The discovery of these 83 starburst galaxies provides critical evidence for the role of small, low-mass galaxies in reionization. Unlike massive galaxies, which were less common in the early universe, these tiny galaxies were abundant at redshift 7, making up a significant fraction of the galaxy population. Their starburst activity, characterized by rapid star formation, produced the intense ultraviolet light needed to strip electrons from hydrogen atoms, ionizing the interstellar medium.

The NIRSpec data confirmed the presence of doubly ionized oxygen in the 20 galaxies studied in detail, indicating vigorous star formation. The galaxies’ redshifts, ranging from 6.8690 to 6.8717, place them at a cosmic age of approximately 790 million years, aligning with the peak of reionization. These findings, detailed in studies by Bezanson et al. (2024) and Wold et al. (2025), underscore the power of JWST’s instruments to probe the early universe with unprecedented sensitivity.

The Technology Behind the Discovery

The James Webb Space Telescope, launched on December 25, 2021, is a marvel of modern astronomy. Its 6.5-meter gold-coated mirror and suite of infrared instruments, including NIRCam and NIRSpec, enable it to detect faint objects from the universe’s earliest epochs. The UNCOVER program leveraged NIRCam’s ability to capture high-resolution images through filters like F410M, which is sensitive to the infrared-shifted light of doubly ionized oxygen, and NIRSpec’s spectroscopic capabilities to analyze the chemical composition and star-forming activity of these galaxies.

Gravitational lensing by Abell 2744 enhanced JWST’s reach, magnifying the light of distant galaxies by up to 13 times. This natural amplification allowed researchers to detect galaxies that would otherwise be too faint, revealing their structure and composition in exquisite detail. The composite NIRCam image, created using filters F200W (blue), F410M (green), and F444W (red), highlights the vibrant star-forming regions of these galaxies, with white diamonds marking the 20 selected for deeper study.

Implications for Cosmic Evolution

The discovery of these starburst galaxies reshapes our understanding of the early universe. Previous studies suggested that massive galaxies or supermassive black holes might have driven reionization, but the abundance and efficiency of these small galaxies point to a different narrative. Their ability to produce and release ultraviolet light in large quantities suggests that they were the dominant force in clearing the cosmic fog, enabling the universe to become transparent.

These findings also have implications for galaxy formation. The small size and rapid star formation of these galaxies indicate that they were compact, dense systems, possibly resembling modern-day dwarf galaxies or green peas. Their starburst activity likely fueled the growth of larger galaxies over time, as mergers and accretion built more massive systems. By studying these early galaxies, astronomers can trace the origins of the Milky Way and other modern galaxies, shedding light on the processes that shaped cosmic structure.

Future Research Directions

The discovery of these 83 starburst galaxies is just the beginning. The UNCOVER team plans to conduct further spectroscopic observations to characterize the remaining galaxies and refine their properties. Key questions include:

• Chemical Composition: Detailed spectra could reveal the presence of elements like nitrogen or carbon, providing insights into the early stars’ makeup.

• Star Formation Rates: Quantifying the rate of star formation could clarify the galaxies’ contribution to reionization.

• Population Studies: Identifying more galaxies at similar redshifts could confirm their abundance and role in cosmic evolution.

• Environmental Impact: Studying the galaxies’ surroundings could reveal how their ultraviolet light interacted with the interstellar medium.

Future JWST programs, such as the JWST Advanced Deep Extragalactic Survey (JADES), will build on these findings, targeting other galaxy clusters and deep fields to uncover more early galaxies. Ground-based observatories like the Atacama Large Millimeter/submillimeter Array (ALMA) could complement these efforts by studying the gas and dust content of these systems.

JWST’s Broader Impact

The James Webb Space Telescope, a collaboration between NASA, the European Space Agency (ESA), and the Canadian Space Agency (CSA), is revolutionizing our view of the cosmos. Since its first science images in July 2022, JWST has delivered groundbreaking discoveries, from the earliest galaxies to exoplanet atmospheres. Its ability to probe the infrared spectrum allows it to see through cosmic dust and observe objects too faint or distant for other telescopes, such as the Hubble Space Telescope.

The UNCOVER program exemplifies JWST’s power to address fundamental questions about the universe’s history. By combining imaging and spectroscopy with gravitational lensing, researchers can study the earliest galaxies with unprecedented detail, unlocking secrets about the Cosmic Dawn—the period when the first stars and galaxies formed.

Inspiring the Next Generation

This discovery is not just a scientific triumph; it’s a chance to inspire awe and curiosity. NASA’s outreach efforts, including resources like the “Related for Kids” section, make these findings accessible to young learners, fostering interest in astronomy and science. The vibrant images of Abell 2744, with its sparkling galaxies and diamond-marked starbursts, capture the imagination, inviting people of all ages to explore the universe’s wonders.

For more on this discovery and JWST’s mission, visit NASA’s official Webb page. The story of these tiny galaxies reminds us that even the smallest players can have an outsized impact on the cosmic stage.