Growth of “upside-down” galaxies observed in the early universe

Astronomers used NASA/ESA’s James Webb Space Telescope (JWST) to observe the “upside-down” growth of a galaxy in the early universe, just 700 million years after the Big Bang.

This galaxy is a hundred times smaller than the Milky Way, but it is surprisingly mature for such an early 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 big city, this galaxy begins to expand, with star formation accelerating around the edges.

This is the first detection of reverse galactic growth. Until Webb, it was not possible to study the growth of galaxies this early in the history of the universe.

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 gas clouds into structures complexities that we observe today. The results are reported in the journal Natural astronomy.

“The question of how galaxies evolve over cosmic time is an important question in astrophysics,” said co-senior author Dr Sandro Tacchella of the Cavendish Laboratory in Cambridge.

“We’ve had a lot of great data from the last ten million years and from galaxies in our corner of the universe, but now with Webb we can get observational data from billions of years back, probing the first billion years of cosmic space history, which opens up all sorts of new questions.

The galaxies we observe today grow via two main mechanisms: they either attract or accumulate 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 that astronomers hope to address with Webb.

“You would expect galaxies to start small as gas clouds collapse under their own gravity, forming very dense star cores and eventually black holes,” Tacchella said.

“As the galaxy grows and star formation increases, it’s a bit like a figure skater spinning: as the skater pulls on his arms, he gains momentum and spins more and more Fast Galaxies are somewhat similar, gas accumulating later from larger ones and greater distances cause the galaxy to rotate, which is why they often form spiral or disk shapes.

This galaxy, observed as part of the JADES (JWST Advanced Extragalactic Survey) collaboration, actively forms stars in the early universe. Its core is very dense and, despite its relatively young age, its density is similar to that of today’s massive elliptical galaxies, which have 1,000 times more stars. Most star formation occurs further from the core, with a star-forming “cluster” even further away.

Star formation activity increases sharply toward the periphery, as star formation expands and the galaxy grows. This type of growth had been predicted by theoretical models, but with Webb it is now possible to observe it.

“One of the many reasons Webb is so transformative for us as astronomers is that we are now able to observe what had previously been predicted through modeling,” said co-author William Baker, who holds a Ph.D. student at 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 compared to older stars, which are then converted into an estimate of stellar mass and star formation rate.

Because the galaxy is so compact, individual images of the galaxy have been “forward modeled” to account for instrumental effects. Using stellar population modeling that includes prescriptions for gas emission and dust absorption, the researchers discovered older stars in the core, while the surrounding disk component undergoes star formation. very active stars.

This galaxy doubles its stellar mass at its periphery approximately every 10 million years, which is very fast: the Milky Way only doubles its mass every 10 billion years.

The density of the galactic core, along with the high rate of star formation, suggests that this young galaxy is rich in the gas it needs to form new stars, which may reflect different conditions in the early universe.

“Of course, it’s only one galaxy, so we need to know what other galaxies were doing at the time,” Tacchella said.

“Were all galaxies like this? We are currently analyzing similar data from other galaxies. By examining different galaxies across cosmic time, we may be able to reconstruct the growth cycle and demonstrate how galaxies grow to their current size.”