In extragalactic astrophysics, the processes that shape how galaxies evolve and their different structures still need to be better understood. A new study using the James Webb Space Telescope (JWST) has reported the observation of a surprisingly mature galaxy from the early Universe.
The JADES-GS+53.18343−27.79097 galaxy formed within the first 700 million years after the Big Bang and is about one hundred times smaller than the Milky Way. Despite its small size, it appears mature for such an early stage in the Universe, with a dense collection of stars at its center and a less dense area in its outskirts. Interestingly, star formation is picking up speed in the galaxy’s outer regions.
This star-forming galaxy has a total stellar mass of 400 million solar masses and consists of three main parts: a compact core with a half-light radius of less than 100 parsecs, a star-forming disk that extends about 400 parsecs, and a star-forming clump. Each component has its own unique star-formation history.
The density of stars at the center of this galaxy is comparable to that of some of the most massive elliptical galaxies we see today, even though this galaxy is 1,000 times less massive overall.
This marks the earliest detection of “inside-out” growth in a galaxy. Before the James Webb Space Telescope (JWST), studying how galaxies developed so early in the Universe’s history was impossible.
While Webb’s images show a moment in time, the researchers believe that examining similar galaxies can provide insights into how they evolved from clouds of gas into the complex structures we see today.
Co-lead author Dr. Sandro Tacchella from Cambridge’s Cavendish Laboratory said, “We’ve 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.”
Galaxies today grow mainly in two ways: by pulling in gas to form new stars or by merging with smaller galaxies. Astronomers are interested in figuring out whether these mechanisms were also at work in the early Universe, and they hope to answer this question using the James Webb Space Telescope (JWST).
Dr. Tacchella explains that galaxies are expected to start small as gas clouds collapse under their own gravity, forming dense cores of stars and possibly black holes. As a galaxy grows and star formation increases, it behaves like a spinning figure skater: when the skater pulls in their arms, they spin faster. Similarly, as galaxies accrete gas from farther distances, they gain momentum and often take on spiral or disk shapes.
The galaxy studied, part of the JADES (JWST Advanced Extragalactic Survey) collaboration, is actively forming stars in the early Universe. It has a very dense core, which, despite its youth, has a density similar to that of massive present-day elliptical galaxies that contain 1,000 times more stars. Most star formation occurs farther from the core, with a star-forming “clump” located even further out.
Interestingly, star formation activity is increasing towards the outskirts of the galaxy as it grows. Theoretical models predicted this type of growth, but astronomers can now observe it directly with the JWST.
Co-author William Baker, a PhD student at the Cavendish said, “One of the many reasons that Webb is so transformational to us as astronomers is that we’re now able to observe what had previously been predicted through modeling. It’s like being able to check your homework.”
Using the James Webb Space Telescope (JWST), researchers analyzed the light emitted by the galaxy at various wavelengths to estimate the number of younger versus older stars. This information allowed them to calculate the galaxy’s stellar mass and star formation rate.
Because the galaxy is so compact, the researchers used “forward modeling” to correct for any instrumental effects in the images. By applying stellar population modeling that accounts for gas emissions and dust absorption, they discovered that the galaxy’s core contains older stars while the surrounding disc forms new stars.
Remarkably, this galaxy doubles its stellar mass in its outer regions roughly every 10 million years, which is extremely fast compared to the Milky Way, which only doubles its mass every 10 billion years.
The high density of the galactic core and the rapid star formation rate indicate that this young galaxy has a plentiful supply of gas for creating new stars. This finding may suggest that conditions in the early Universe were quite different from those we see today.
Tacchella said, “We’re 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., Tacchella, S., Johnson, B.D. et al. A core in a star-forming disc as evidence of inside-out growth in the early Universe. Nat Astron (2024). DOI: 10.1038/s41550-024-02384-8