Astronomers have used the NASA/ESA James Webb Space Telescope (JWST) to observe the ‘inside-out’ growth of a galaxy in the early universe, only 700 million years after the Big Bang. This galaxy, one hundred times smaller than the Milky Way, is surprisingly mature for its early stage in the universe. It has a dense core of stars like a large city, which becomes less dense in the galactic ‘suburbs’. Star formation is accelerating in the outskirts, similar to a city sprawl. This discovery represents the earliest-ever detection of inside-out galactic growth, made possible by the capabilities of the Webb telescope.
Until now, studying galaxy growth so early in the universe’s history had not been feasible. The researchers, led by the University of Cambridge, obtained images with Webb that provide a snapshot in time. By studying similar galaxies, researchers hope to understand how galaxies transform from gas clouds to the complex structures observed today. This groundbreaking research is reported in the journal Nature Astronomy, shedding light on the evolution of galaxies over cosmic time and expanding our understanding of the first billion years of cosmic history.
Galaxies observed today grow through two main mechanisms: accreting gas to form new stars or merging with smaller galaxies. Astronomers are eager to determine which mechanisms were at work in the early universe, a question they hope to answer with the help of the Webb telescope. The formation of galaxies is likened to a spinning figure skater, gathering momentum as gas accretes from larger distances, spinning the galaxy up and leading to the formation of spiral or disc shapes. By observing this early galaxy, researchers are gaining valuable insights into the growth processes at work in the early universe.
The galaxy observed by the JADES collaboration is actively forming stars and has a highly dense core, similar in density to present-day massive elliptical galaxies. Most star formation is occurring away from the core, with a star-forming ‘clump’ even further out. This growth pattern had been predicted by theoretical models, but now, with Webb, researchers are able to observe it directly. The rising star formation activity toward the outskirts indicates the galaxy is spreading out and growing in size, providing valuable data for astronomers to understand the growth of galaxies over time.
Using information extracted from the light emitted by the galaxy at different wavelengths, researchers estimated the number of younger stars versus older stars, which provides insight into the stellar mass and star formation rate. By forward modelling individual images of the galaxy to account for instrumental effects, researchers found older stars in the core and active star formation in the surrounding disc component. This young galaxy is doubling its stellar mass in the outskirts every 10 million years, a rapid rate compared to the Milky Way’s mass doubling every 10 billion years. The high density of the galactic core and the rapid star formation rate suggest rich gas conditions in the early universe.
To gain a comprehensive understanding of galaxy growth in the early universe, researchers are now analyzing data from other galaxies. By studying different galaxies across cosmic time, they aim to reconstruct the growth cycle and demonstrate how galaxies evolve to their present size. This research represents a significant step forward in astrophysics, using the capabilities of the Webb telescope to uncover the mysteries of galaxy evolution in the earliest stages of the universe. The transformative nature of this research highlights the importance of observational data in expanding our knowledge of the cosmos.