Astronomers analyzing data from NASA’s James Webb Space Telescope have uncovered hundreds of tiny galaxies that played key roles in a cosmic makeover that turned the early universe into the one we know today.
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. Image Credit: NASA/ESA/CSA/Bezanson et al. 2024 and Wold et al. 2025
When it comes to producing ultraviolet light, these small galaxies punch well above their weight. 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.
Isak Wold, Assistant Research Scientist, NASA Goddard Space Flight Center
Wold presented his findings at the American Astronomical Society's 246th conference in Anchorage, Alaska. The study makes use of previously obtained images from Webb's NIRCam (Near-Infrared Camera) instrument as well as fresh observations taken with its NIRSpec (Near-Infrared Spectrograph).
By searching through Webb images taken as part of the UNCOVER (Ultradeep NIRSpec and NIRCam ObserVations before the Epoch of Reionization) observing program, which was headed by Rachel Bezanson at the University of Pittsburgh in Pennsylvania, Wold and his Goddard colleagues Sangeeta Malhotra and James Rhoads found the tiny galaxies.
Abell 2744, sometimes referred to as Pandora's cluster, is a massive galaxy cluster in the southern constellation Sculptor that is around 4 billion light-years distant. The project mapped this cluster. Webb's already significant reach is increased by the gravitational lens created by the cluster's mass, which magnifies far-off sources.
The universe was shrouded in a mist of neutral hydrogen gas for a large portion of its initial billion years. This gas has lost its electrons and is now ionized.
For a long time, astronomers have been wondering whether types of objects such as large galaxies, tiny galaxies, or supermassive black holes in active galaxies were most responsible for this change, which they call reionization. NASA's Webb was created with the express purpose of answering important questions regarding this significant shift in the universe’s history.
Small galaxies going through a period of intense star formation may have had a significant impact, according to recent research. These galaxies are extremely uncommon nowadays, accounting for only 1% of all galaxies. However, reionization was well advanced when the universe was around 800 million years old, a period known to astronomers as redshift 7.
In NIRCam images of the cluster, the scientists looked for tiny galaxies of the correct cosmic age that had starbursts, which are indications of extreme star formation.
Low-mass galaxies gather less neutral hydrogen gas around them, which makes it easier for ionizing ultraviolet light to escape. 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.
James Rhoads, Research Astrophysicist, NASA Goddard Space Flight Center
Strong sources of a particular wavelength of light, which indicates the existence of high-energy processes and is released by oxygen atoms that have lost two electrons, were sought after.
The green glow from doubly ionized oxygen was first released as visible light in the early universe. As it traveled across the expanding universe, it was stretched into the infrared and finally arrived at Webb’s instruments.
Using this method, 83 small starburst galaxies were discovered to be 800 million years old, or around 6% of the universe's current age of 13.8 billion years. Twenty of them were chosen by the researchers for further examination with 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. But we are able to detect them because of our novel sample selection technique combined with gravitational lensing,” said Malhotra.
About 25% of the ionizing ultraviolet light from galaxies like green peas in the modern universe is released into the surrounding space. Wold and his team’s low-mass starburst galaxies can provide all of the ultraviolet light required to change the neutral hydrogen in the universe into its ionized state if they emit a similar amount.