Astronomers recently captured the earliest glimpse of “inside-out” galaxy growth, offering a new perspective on how galaxies in the infant universe developed. Galaxy NGC 1549, born just 700 million years after the Big Bang, provides a rare window into a critical period of cosmic development. While only a fraction of the Milky Way’s size, this galaxy shows a surprisingly sophisticated structure for such a young system, shedding light on the processes driving early galaxy creation.
Discovered during the James Webb Space Telescope Advanced Deep Extragalactic Survey (JADES), the new find reveals a pattern of growth similar to that seen in much older galaxies today: a densely packed core of stars at the center, with new stars rapidly forming in the outer regions. However, they noticed something totally unexpected too: star formation was concentrated on the galaxy’s outer edges rather than the core, hence the term “inside-out” growth. This process had been theorized for years but never directly observed in such early cosmic history.
“The question of how galaxies evolve over cosmic time is an important one in astrophysics,” said co-lead author Sandro Tacchella from Cambridge’s Cavendish Laboratory. “We’ve had lots of excellent data for the last ten million years and for galaxies in our corner of the universe, but now with Webb, we can get observational data from billions of years back in time, probing the first billion years of cosmic history, which opens up all kinds of new questions.”
Galaxies in the early universe were thought to grow by accumulating gas, forming stars either through direct accretion or merging with smaller galaxies. Over time, this gas collapses under its own gravity and forms a dense core of stars. Those then expand outward as gas from more distant regions spins up and contributes to star formation in the outskirts. Cornell University had an inkling this was happening in the late 90s, but the new study confirms the former theory.
“You expect galaxies to start small as gas clouds collapse under their own gravity, forming very dense cores of stars and possibly black holes,” said Tacchella. “As the galaxy grows and star formation increases, it’s sort of like a spinning figure skater: as the skater pulls in their arms, they gather momentum, and they spin faster and faster. Galaxies are somewhat similar, with gas accreting later from larger and larger distances spinning the galaxy up, which is why they often form spiral or disc shapes.”
The galaxy observed by JADES stands out for its compact and dense core, which has a half-light radius of just 144 parsecs (about 470 light-years). Despite its small size, the core is comparable in density to the massive elliptical galaxies astronomers observe today. Yet, it contains far fewer stars—around 1,000 times fewer.
A star-forming disc encircling this core stretches to 468 parsecs, with a prominent star-forming clump located even farther out. The real action is in this clump and the disc, where the star birth rate doubles the galaxy’s mass every 10 million years. In comparison, the Milky Way’s mass doubles every 10 billion years.
One of the most intriguing aspects of this discovery is how JWST’s instruments enabled researchers to extract detailed information about the galaxy’s star formation. The team used light from different wavelengths to estimate the number of young versus old stars, determining how the galaxy’s stellar population is distributed. Their findings show that the outer regions undergo intense star formation while older stars populate the core. This means that the galaxy, though young, is already building complexity from its core outward, with rapid growth occurring in the outer regions.
William Baker, a PhD student at Cavendish and co-author of the study, highlighted the value of JWST in bridging the gap between theoretical predictions and real-world observations.
“One of the many reasons that Webb is so transformational to us as astronomers is that we’re now able to observe what had previously been predicted through modeling,” he said. “It’s like being able to check your homework.”
The discovery of this galaxy during the Epoch of Reionization—when the first galaxies and stars formed—adds to our understanding of how galaxies in the early universe behaved. While this galaxy is just one example, its structure and star formation patterns provide a foundation for studying other galaxies from this critical period. Researchers are now investigating whether this inside-out growth pattern was common during the early universe or if different environments influenced galaxy formation in unique ways.
“Of course, this is only one galaxy, so we need to know what other galaxies at the time were doing,” said Tacchella. “Were all galaxies like this one? We’re now analyzing similar data from other galaxies. By looking at different galaxies across cosmic time, we may be able to reconstruct the growth cycle and demonstrate how galaxies grow to their eventual size today.”
The study was published in Nature Astronomy.
