NASA’s Webb ‘UNCOVERs’ Galaxy Population Driving Cosmic Renovation

NASA · June 11

5 Min Read

NASA’s Webb ‘UNCOVERs’ Galaxy Population Driving Cosmic Renovation

Composite Webb NIRCam image of galaxy cluster Abell 2744, showing the locations of young starburst galaxies. White and yellow galaxies of various sizes and shapes appear against the blackness of space. Two bright stars in our own galaxy display prominent six-spike diffraction patterns with bluish rays, visible at center left and lower left. Superimposed on the infrared image are 20 white diamonds, each representing a young galaxy investigated in greater detail.

White diamonds show the locations of 20 of the 83 young, low-mass, starburst galaxies found in infrared images of the giant galaxy cluster Abell 2744. Full image and description shown below.

Credits:
NASA/ESA/CSA/Bezanson et al. 2024 and Wold et al. 2025

Astronomers using data from NASA’s James Webb Space Telescope have identified dozens of small galaxies that played a starring role in a cosmic makeover that transformed the early universe into the one we know today.

“When it comes to producing ultraviolet light, these small galaxies punch well above their weight,” said Isak Wold, an assistant research scientist at Catholic University of America in Washington and NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Our analysis of these tiny but mighty galaxies is 10 times more sensitive than previous studies, and shows they existed in sufficient numbers and packed enough ultraviolet power to drive this cosmic renovation.”

Wold discussed his findings Wednesday at the 246th meeting of the American Astronomical Society in Anchorage, Alaska. The study took advantage of existing imaging collected by Webb’s NIRCam (Near-Infrared Camera) instrument, as well as new observations made with its NIRSpec (Near-Infrared Spectrograph) instrument.

Image A: Webb search finds dozens of tiny, young star-forming galaxies

Animation showing the locations of young, low-mass, starburst galaxies around galaxy cluster Abell 2744. White and yellow galaxies of various sizes and shapes appear against the blackness of space. Two bright stars in our own galaxy display prominent six-spike diffraction patterns with bluish rays, visible at center left and lower left. Then 20 white diamonds sweep across the image. One diamond enlarges to reveal an image of a young, low-mass, star-forming galaxy. It looks like a green oval against a red and green checked background. The enlarged image then shrinks back, and the diamonds sweep away. The sequence loops.

Symbols mark the locations of young, low-mass galaxies bursting with new stars when the universe was about 800 million years old. Using a filter sensitive to such galaxies, NASA’s James Webb Space Telescope imaged them with the help of a natural gravitational lens created by the massive galaxy cluster Abell 2744. In all, 83 young galaxies were found, but only the 20 shown here (white diamonds) were selected for deeper study. The inset zooms into one of the galaxies.
Download high-resolution images from NASA’s Scientific Visualization Studio

NASA/ESA/CSA/Bezanson et al. 2024 and Wold et al. 2025

The tiny galaxies were discovered by Wold and his Goddard colleagues, Sangeeta Malhotra and James Rhoads, by sifting through Webb images captured as part of the UNCOVER (Ultradeep NIRSpec and NIRCam ObserVations before the Epoch of Reionization) observing program, led by Rachel Bezanson at the University of Pittsburgh in Pennsylvania.

The project mapped a giant galaxy cluster known as Abell 2744, nicknamed Pandora’s cluster, located about 4 billion light-years away in the southern constellation Sculptor. The cluster’s mass forms a gravitational lens that magnifies distant sources, adding to Webb’s already considerable reach.

Image B: Galaxy cluster helps reveal young, low-mass galaxies bursting with stars

White diamonds show the locations of 20 of the 83 young, low-mass, starburst galaxies found in infrared images of the giant galaxy cluster Abell 2744. This composite incorporates images taken through three NIRCam filters (F200W as blue, F410M as green, and F444W as red). The F410M filter is highly sensitive to light emitted by doubly ionized oxygen — oxygen atoms that have been stripped of two electrons — at a time when reionization was well underway. Emitted as green light, the glow was stretched into the infrared as it traversed the expanding universe over billions of years. The cluster’s mass acts as a natural magnifying glass, allowing astronomers to see these tiny galaxies as they were when the universe was about 800 million years old.

NASA/ESA/CSA/Bezanson et al. 2024 and Wold et al. 2025

For much of its first billion years, the universe was immersed in a fog of neutral hydrogen gas. Today, this gas is ionized — stripped of its electrons. Astronomers, who refer to this transformation as reionization, have long wondered which types of objects were most responsible: big galaxies, small galaxies, or supermassive black holes in active galaxies. As one of its main goals, NASA’s Webb was specifically designed to address key questions about this major transition in the history of the universe.

Recent studies have shown that small galaxies undergoing vigorous star formation could have played an outsized role. Such galaxies are rare today, making up only about 1% of those around us. But they were abundant when the universe was about 800 million years old, an epoch astronomers refer to as redshift 7, when reionization was well underway.

The team searched for small galaxies of the right cosmic age that showed signs of extreme star formation, called starbursts, in NIRCam images of the cluster.

“Low-mass galaxies gather less neutral hydrogen gas around them, which makes it easier for ionizing ultraviolet light to escape,” Rhoads said. “Likewise, starburst episodes not only produce plentiful ultraviolet light — they also carve channels into a galaxy’s interstellar matter that helps this light break out.”

Image C: A deeper look into small, young, star-forming galaxies during reionization

Close-up of Abell 2744 showing detailed information on three young starburst galaxies. Three columns of images and information appear on a black background. At left is an infrared image showing white and yellow galaxies of various shapes and sizes, with three green diamonds superimposed on it. Light gray shading extends from each diamond to a square image in the center column. The top image is labeled 41038, the middle one is 41028, and at bottom is 41006. Each image shows a green blob on a red and green checked background, while the middle and bottom images include a large white blob as well. Next to each image is textual information. For the top image, it reads “Magnified 13 X, z = 6.8690, Stellar mass 10 million Suns”; for the middle image, “Magnified 11 X, z = 6.8697, Stellar mass 2 million Suns”; for the bottom image, “Magnified 3 X, z= 6.8717, Stellar mass 160 million Suns.” At right is a column of line graphs and colored bars representing a spectrum of each galaxy. The colored bars align with peaks in a squiggly line running horizontally across each graph. There is one blue bar, labelled “Hydrogen,” and two green bars, labeled “Oxygen,” in each graph. The vertical axis of the graphs reads “Brightness,” while the horizontal axis reads “Wavelength (µm),” with numbers ranging from 3.82 to 3.94.

At left is an enlarged infrared view of galaxy cluster Abell 2744 with three young, star-forming galaxies highlighted by green diamonds. The center column shows close-ups of each galaxy, along with their designations, the amount of magnification provided by the cluster’s gravitational lens, their redshifts (shown as z — all correspond to a cosmic age of about 790 million years), and their estimated mass of stars. At right, measurements from NASA’s James Webb Space Telescope’s NIRSpec instrument confirm that the galaxies produce strong emission in the light of doubly ionized oxygen (green bars), indicating vigorous star formation is taking place.

NASA/ESA/CSA/Bezanson et al. 2024 and Wold et al. 2025

The astronomers looked for strong sources of a specific wavelength of light that signifies the presence of high-energy processes: a green line emitted by oxygen atoms that have lost two electrons. Originally emitted as visible light in the early cosmos, the green glow from doubly ionized oxygen was stretched into the infrared as it traversed the expanding universe and eventually reached Webb’s instruments.

This technique revealed 83 small starburst galaxies as they appear when the universe was 800 million years old, or about 6% of its current age of 13.8 billion years. The team selected 20 of these for deeper inspection using NIRSpec.

“These galaxies are so small that, to build the equivalent stellar mass of our own Milky Way galaxy, you’d need from 2,000 to 200,000 of them,” Malhotra said. “But we are able to detect them because of our novel sample selection technique combined with gravitational lensing.”

Image D: Tiny but mighty galaxy helped clear cosmic fog

Enlarged view of a single young starburst galaxy. A pixelated green oval extending from upper left to lower right sits in the middle of a red and green checked background. A large white blob is visible at center right.

One of the most interesting galaxies of the study, dubbed 41028 (the green oval at center), has an estimated stellar mass of just 2 million Suns — comparable to the masses of the largest star clusters in our own Milky Way galaxy.

NASA/ESA/CSA/Bezanson et al. 2024 and Wold et al. 2025

Similar types of galaxies in the present-day universe, such as green peas, release about 25% of their ionizing ultraviolet light into surrounding space. If the low-mass starburst galaxies explored by Wold and his team release a similar amount, they can account for all of the ultraviolet light needed to convert the universe’s neutral hydrogen to its ionized form.

The James Webb Space Telescope is the world’s premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).

To learn more about Webb, visit:

https://science.nasa.gov/webb

By Francis Reddy
NASA’s Goddard Space Flight Center, Greenbelt, Md.