Though 100 times smaller than the Milky Way, the galaxy observed appears remarkably advanced for its age. It features a dense central core of stars, surrounded by less dense outer regions where star formation is accelerating - a process likened to the sprawl of a growing city.
"This is the first time we've seen inside-out growth in such an early galaxy," said Dr. Sandro Tacchella from Cambridge's Cavendish Laboratory. "Before Webb, we couldn't observe galaxy growth from this period." The study, led by the University of Cambridge, is detailed in 'Nature Astronomy'.
Astronomers have long studied galaxy growth in the modern universe, but the JWST now allows observations going back billions of years. The galaxies we see today typically grow by accreting gas to form stars or by merging with smaller galaxies. "Understanding how galaxies evolved during this early cosmic time is critical," added Tacchella.
Tacchella explained that galaxies initially form as gas clouds collapse under gravity, forming dense star clusters. As more gas is drawn in, the galaxy spins faster, often leading to the formation of spiral or disc shapes. This particular galaxy, part of the JADES (JWST Advanced Extragalactic Survey) collaboration, is still forming stars and features a core as dense as present-day massive elliptical galaxies, even though it is much younger.
Most of the star formation is occurring away from the core, and the outskirts of the galaxy are expanding rapidly. "Webb allows us to observe what theoretical models had predicted," said co-author William Baker, a PhD student at Cambridge. "It's like being able to check your homework."
By analyzing the light from the galaxy at various wavelengths, researchers estimated the ages of the stars, the stellar mass, and the rate of star formation. The core consists of older stars, while the surrounding disc experiences rapid star formation. The galaxy's outskirts double their stellar mass approximately every 10 million years, far faster than the Milky Way, which doubles its mass every 10 billion years.
The dense core and rapid star formation suggest the galaxy is rich in gas, reflecting different conditions in the early universe. "This is just one galaxy," said Tacchella. "We need to study others from the same period to understand whether this pattern of growth was common. By doing so, we can potentially reconstruct how galaxies evolve to the sizes we see today."
Research Report:A core in a star-forming disc as evidence of inside-out growth in the early Universe
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