Big Bang city: Ancient galaxy 100x smaller than Milky Way, grows 1,000x faster

Astronomers have made a groundbreaking observation using the NASA/ESA James Webb Space Telescope (JWST), uncovering a galaxy from just 700 million years after the Big Bang that demonstrates an ‘inside-out’ growth pattern.

This early universe galaxy, though one hundred times smaller than the Milky Way, shows surprising maturity. Its outskirts have a densely packed core of stars and active star formation.

This phenomenon marks the earliest recorded example of inside-out galactic growth, a discovery made possible only through Webb’s unprecedented capabilities.

Like a bustling city with a dense urban center and sprawling suburbs, this galaxy is compact at its core, where most of its stars are tightly clustered, while newer stars are forming at an accelerated rate in the outer regions.

City of stars?

This process of star formation in the outer reaches signals that the galaxy is expanding. The findings, part of the JWST Advanced Extragalactic Survey (JADES), provide fresh insights into how galaxies grew and evolved in the universe’s infancy, allowing astronomers to explore new dimensions of cosmic history.

While this observation provides a snapshot of a single moment in time, researchers, led by a team from the University of Cambridge, believe that studying similar galaxies could reveal how these celestial structures evolved from simple gas clouds into the intricate galaxies we observe today.

“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.”

Dr. Tacchella likened the growth of galaxies to a spinning figure skater: as gas is pulled inward from great distances, the galaxy spins faster, often leading to the formation of spiral or disc shapes.

This galaxy, though young, has a highly dense core reminiscent of present-day massive elliptical galaxies, which typically contain 1,000 times more stars. Interestingly, most of the star formation is not happening in the core but rather in the outskirts, where a clump of stars is rapidly forming. The growth pattern observed here had been previously predicted by theoretical models, but this is the first time scientists have been able to witness it directly.

Double the mass in 10 million years

Using JWST’s advanced technology, the researchers analyzed the light emitted by the galaxy at different wavelengths to estimate the ages of the stars, their masses, and the rate of star formation. Due to the compactness of the galaxy, the images had to be carefully modeled to account for potential distortions caused by the instrument.

By incorporating models of stellar populations, gas emission, and dust absorption, they discovered that the older stars are concentrated in the core, while the surrounding disc is actively forming new stars. Remarkably, this galaxy doubles its stellar mass in the outskirts approximately every 10 million years—a rate that vastly outpaces that of the Milky Way, which takes 10 billion years to achieve the same mass doubling.

The dense core and rapid star formation suggest that this galaxy is rich in the gas necessary to fuel its growth. This could indicate that conditions in the early universe were markedly different from what we see today, providing ample resources for such rapid star formation.

Despite the significance of this discovery, Tacchella stressed the need for further study: “Of course, this is only one galaxy, so we need to know what other galaxies at the time were doing. 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.”

This discovery marks a significant step forward in our understanding of how galaxies formed and evolved in the early universe. As more data from JWST becomes available, astronomers hope to continue unraveling the mysteries of galactic growth and the forces that shaped the universe’s earliest stars and structures.

The results are reported in the journal Nature Astronomy .