Scientists have utilized the NASA/ESA James Webb Space Telescope (JWST) to observe a galaxy’s ‘inside-out’ growth during the early universe, a mere 700 million years after the Big Bang.
This galaxy is around one hundred times smaller than the Milky Way yet displays an unexpected level of maturity for such an early stage in the universe. Much like a bustling city, it has a crowded center filled with stars while its outskirts are less populated. Additionally, akin to urban sprawl, new star formation is intensifying on the fringes.
This marks the earliest known observation of inside-out galactic growth. Prior to Webb, studying galaxy development at such an early cosmic time was not feasible. While the images captured by Webb represent a moment in time, researchers — led by the University of Cambridge — assert that examining similar galaxies could enhance our understanding of how they evolve from gas clouds to the intricate structures we see today. These findings are published in the journal Nature Astronomy.
“Understanding the evolution of galaxies over cosmic time is a crucial topic in astrophysics,” stated co-lead author Dr. Sandro Tacchella from the Cavendish Laboratory at Cambridge. “We had great data for the last ten million years and for galaxies nearby, but now, with Webb’s help, we can gather observational data from billions of years ago, exploring the first billion years of cosmic history, raising countless new inquiries.”
The galaxies we witness today expand through two primary methods: they either acquire gas to form new stars or grow through merging with smaller galaxies. Whether these different processes were in play during the early universe remains an ongoing inquiry that astronomers intend to investigate with Webb.
“Initially, galaxies are expected to form from collapsing gas clouds under their own gravity, creating very dense star cores and potentially black holes,” explained Tacchella. “As they grow and star formation quickens, it’s comparable to a figure skater pulling in their arms to spin faster. Similarly, as more gas is drawn from further distances later, galaxies speed up, commonly resulting in spiral or disc shapes.”
This galaxy, part of the JADES (JWST Advanced Extragalactic Survey) project, is actively generating stars during the early universe. Its core is highly dense and, despite its youth, has a density comparable to today’s massive elliptical galaxies, which have a thousand times more stars. The majority of star formation occurs farther from the core, with an additional star-forming ‘clump’ even further out.
Star formation is significantly increasing toward the outer regions, indicating that as the galaxy expands, star generation is spreading outward. This growth pattern was theorized previously but is now observable thanks to Webb.
“One of the many groundbreaking aspects of Webb for us astronomers is our ability to witness phenomena that were only predicted in models before,” remarked co-author William Baker, a PhD student at the Cavendish. “It’s akin to being able to verify your homework.”
With Webb, the team gathered data from the light emitted by the galaxy across different wavelengths, which they used to estimate the ratio of younger stars to older stars, converting that into assessments of stellar mass and star formation rates.
Due to the compact nature of the galaxy, individual images were ‘forward modeled’ to account for instrument effects. By employing stellar population modeling that incorporates aspects of gas emission and dust absorption, researchers identified older stars in the core, while the surrounding disc is experiencing very active star formation. Remarkably, this galaxy doubles its stellar mass on the outskirts approximately every 10 million years—a rate incredibly fast compared to the Milky Way, which takes around 10 billion years to double its mass.
The high density of the galactic core and significant star formation rate indicate that this young galaxy possesses ample gas to create new stars, potentially reflecting the different conditions present in the early universe.
“Naturally, this is just one galaxy, so we need to explore the behavior of other galaxies from the same time period,” Tacchella noted. “Did all galaxies exhibit similar characteristics? We are currently analyzing comparable data from other galaxies. By examining various galaxies throughout cosmic time, we could reconstruct the growth patterns and show how galaxies reach their current dimensions.”
