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 is one hundred times smaller than the Milky Way, but is surprisingly mature for so early in the universe. Like a large city, this galaxy has a dense collection of stars at its core but becomes less dense in the galactic 'suburbs'. And like a large city, this galaxy is starting to sprawl, with star formation accelerating in the outskirts.
This is the earliest-ever detection of inside-out galactic growth. Until Webb, it had not been possible to study galaxy growth so early in the universe's history. Although the images obtained with Webb represent a snapshot in time, the researchers, led by the University of Cambridge, say that studying similar galaxies could help us understand how they transform from clouds of gas into the complex structures we observe today. The results are reported in the journal Nature Astronomy.
"The question of how galaxies evolve over cosmic time is an important one in astrophysics," said co-lead author Dr Sandro Tacchella from Cambridge's Cavendish Laboratory. "We've had lots of excellent data for the last ten million years and for 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."
The galaxies we observe today grow via two main mechanisms: either they pull in, or accrete, gas to form new stars, or they grow by merging with smaller galaxies. Whether different mechanisms were at work in the early universe is an open question which astronomers are hoping to address with Webb.
"You expect galaxies to start small as gas clouds collapse under their own gravity, forming very dense cores of stars and possibly black holes," said Tacchella. "As the galaxy grows and star formation increases, it's 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."
This galaxy, observed as part of the JADES (JWST Advanced Extragalactic Survey) collaboration, is actively forming stars in the early universe. It has a highly dense core, which despite its relatively young age, is of a similar density to present-day massive elliptical galaxies, which have 1000 times more stars. Most of the star formation is happening further away from the core, with a star-forming 'clump' even further out.
The star formation activity is strongly rising toward the outskirts, as the star formation spreads out and the galaxy grows. This type of growth had been predicted with theoretical models, but with Webb, it is now possible to observe it.
"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 modelling," said co-author William Baker, a PhD student at the Cavendish. "It's like being able to check your homework."
Using Webb, the researchers extracted information from the light emitted by the galaxy at different wavelengths, which they then used to estimate the number of younger stars versus older stars, which is converted into an estimate of the stellar mass and star formation rate.
Because the galaxy is so compact, the individual images of the galaxy were 'forward modelled' to take into account instrumental effects. Using stellar population modelling that includes prescriptions for gas emission and dust absorption, the researchers found older stars in the core, while the surrounding disc component is undergoing very active star formation. This galaxy doubles its stellar mass in the outskirts roughly every 10 million years, which is very rapid: the Milky Way galaxy doubles its mass only every 10 billion years.
The density of the galactic core, as well as the high star formation rate, suggest that this young galaxy is rich with the gas it needs to form new stars, which may reflect different conditions in the early universe.
"Of course, this is only one galaxy, so we need to know what other galaxies at the time were doing," said Tacchella. "Were all galaxies like this one? We're now analysing 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."
Reference:
William M. Baker, Sandro Tacchella, et al. 'A core in a star-forming disc as evidence of inside-out growth in the early Universe.' Nature Astronomy (2024). DOI: 10.1038/s41550-024-02384-8
