Astronomers have utilized the NASA/ESA James Webb Space Telescope (JWST) to observe the "inside-out" growth of a galaxy from the early universe, just 700 million years after the Big Bang.
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Despite being only one-hundredth the size of the Milky Way, this galaxy exhibits surprising maturity for such an early stage. Much like a busy city, it has a dense concentration of stars at its core, while the outer regions are less populated. Additionally, star formation is accelerating in the galaxy’s outer areas, similar to the urban sprawl seen in large cities.
This marks the earliest observation of inside-out galactic growth. Before Webb, studying galaxies from this early period in the universe’s history was not possible. The JWST images, though capturing only a single moment, offer a glimpse into how these early systems may have evolved from simple gas clouds into the complex structures we observe today.
The question of how galaxies evolve over cosmic time is an important one in astrophysics. We have had lots of excellent data for the last ten million years and 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. Sandro Tacchella, Study Co-Lead Author, Cavendish Laboratory, University of Cambridge
The galaxies we see today grow by either accreting gas to form new stars or merging with smaller galaxies. However, it is still unclear if the same processes occurred in the early universe. Astronomers hope to answer this question using Webb’s powerful capabilities.
You expect galaxies to start small as gas clouds collapse under their gravity, forming very dense cores of stars and possibly black holes. As the galaxy grows and star formation increases, it is sort of like a spinning figure skater: as the skater pulls in their arms, they gather momentum, and they spin faster and faster. Galaxies are somewhat similar, with gas accreting later from larger and larger distances spinning the galaxy up, which is why they often form spiral or disc shapes.
Dr. Sandro Tacchella, Study Co-Lead Author, Cavendish Laboratory, University of Cambridge
This galaxy, observed as part of the JADES (JWST Advanced Extragalactic Survey) collaboration, is actively forming stars. Its dense core, despite its young age, rivals the density of present-day massive elliptical galaxies that contain 1000 times more stars. Most star formation is occurring away from the core, with a distinct star-forming "clump" even further out.
Star formation is rapidly increasing in the galaxy's outskirts as the galaxy expands. This growth, predicted by theoretical models, can now be directly observed thanks to Webb.
One of the many reasons that Webb is so transformational to us as astronomers is that we are now able to observe what had previously been predicted through modeling. It is like being able to check your homework.
William Baker, PhD Student and Study Co-Author, University of Cambridge
Researchers used Webb to analyze the light emitted by the galaxy across various wavelengths to estimate the proportions of younger and older stars. This data allowed them to determine the galaxy's stellar mass and star formation rate.
As a result of the galaxy’s compact size, the images were "forward modeled" to account for instrumental effects. Using stellar population models that include gas emissions and dust absorption, researchers found older stars concentrated in the core, while intense star formation was occurring in the surrounding disk. The galaxy is rapidly growing, doubling its stellar mass in the outskirts every 10 million years—much faster than the Milky Way, which doubles its mass only every 10 billion years.
The dense core and high star formation rate suggest that this galaxy is rich in gas, providing the necessary fuel for star formation, which may reflect unique conditions in the early universe.
Tacchella concluded by stating; “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 are 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.”
Journal Reference:
Baker, W. M., et al. (2024) A core in a star-forming disc is evidence of inside-out growth in the early Universe. Nature Astronomy. doi.org/10.1038/s41550-024-02384-8.