JWST helped see “inside-out” growth of galaxy in the early universe.

Astronomers at the University of Cambridge have been able to observe a galaxy's unique “inside-out” growth in the early universe, using the NASA/ESA James Webb Space Telescope (JWST).

This unusual galaxy, named JADES-GS+53.18343−27.79097, was formed just 700 million years after the Big Bang, which is just 5% of the universe's current age (13.7 billion years), and is significantly smaller than our Milky Way - about one hundred times its size, according to a study published in the journal Nature Astronomy.

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The galaxy displays an unexpected level of maturity for its age, according to co-lead author Dr. Sandro Tacchella from Cambridge’s Cavendish Laboratory. Similar to a bustling city, it has a densely packed core of stars, with decreasing density in its “suburbs.” Additionally, star formation is intensifying in the outer regions, resembling urban sprawl. This marks the earliest detection of inside-out galactic growth, the university stressed in a press release.

Prior to the Webb telescope, studying galaxy formation at such an early stage was unattainable. Although the images captured by Webb provide only a momentary glimpse, researchers working under the umbrella of the University of Cambridge believe that examining similar galaxies will shed light on how they evolve from gas clouds into the intricate structures we see today.

“The evolution of galaxies over cosmic time is a crucial topic in astrophysics,” remarked Tacchella.

“While we've gathered excellent data for the last ten million years and for galaxies in our vicinity, Webb enables us to access observational data from billions of years ago, allowing us to explore the first billion years of cosmic history, which raises numerous new questions.”

Galaxies today typically grow through two primary processes: accreting gas to create new stars or merging with smaller galaxies. Whether these mechanisms operated similarly in the early universe remains an unanswered question that astronomers aim to investigate with Webb.

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“We anticipate galaxies to begin as small gas clouds that collapse under gravity, forming dense cores of stars and potentially black holes,” Tacchella was quoted as saying. “As a galaxy expands and star formation accelerates, it resembles a spinning figure skater: as the skater pulls in their arms, they gain momentum and spin faster. Galaxies exhibit similar behavior, drawing in gas from increasing distances, leading to spiral or disc shapes.”

This particular galaxy, part of the JADES (JWST Advanced Extragalactic Survey) collaboration, is actively forming stars in its early universe setting. Its core is highly dense and, despite its youth, possesses a density comparable to that of present-day massive elliptical galaxies, which typically have a thousand times more stars. Most star formation occurs farther from the core, with a star-forming “clump” – a gas hotspot – located even further out.

Star formation activity is significantly increasing toward the outskirts, indicating that the galaxy is expanding. This growth pattern was predicted by theoretical models but is now observable for the first time thanks to the JWST, empowering astronomers validate their predictions through modeling.

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Using the Webb telescope, researchers analyzed light emitted by the galaxy at various wavelengths to estimate the ratio of younger stars to older stars, translating this into estimations of stellar mass and star formation rates. Stellar population modeling, incorporating gas emission and dust absorption, revealed older stars in the core, while the surrounding disc is experiencing vigorous star formation. Remarkably, this galaxy doubles its stellar mass in its outskirts approximately every 10 million years—significantly faster than the Milky Way, which takes around 10 billion years to double its mass.

The dense core and high star formation rate suggest that this young galaxy possesses abundant gas necessary for new star formation, possibly reflecting the different conditions present in the early universe.

The discovery of JADES-GS+53.18343−27.79097 will enable researchers to understand what other galaxies looked like at that time and to reconstruct the growth cycles in order to illustrate how galaxies reach their current sizes.

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