Cambridge astronomers used James Webb Space Telescope to observe inside-out growth of galaxy only 700 million years after Big Bang

The ‘inside-out’ growth of a galaxy in the early universe – only 700 million years after the Big Bang – has been observed by University of Cambridge astronomers using the James Webb Space Telescope.

One hundred times smaller than the Milky Way, but surprisingly mature given it has been observed so early in the universe, the galaxy is described as like a large city, with a dense collection of stars at its core that becomes less dense in the galactic ‘suburbs’. And it is starting to sprawl, with star formation accelerating in the outskirts.

The earliest detection yet of inside-out galactic growth, it was made possible thanks to the incredible power of the NASA/ESA James Webb Space Telescope. Studying similar galaxies could help us understand how they transform from clouds of gas into the complex structures we now see.

Dr Sandro Tacchella, from Cambridge’s Cavendish Laboratory, co-lead author of a study reported in the journal Nature Astronomy, said: “The question of how galaxies evolve over cosmic time is an important one in astrophysics. We’ve had lots of excellent data for the last 10 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.”

Two main mechanisms are responsible for the growth of galaxies we observe today. Either they pull in, or ‘accrete’, gas to form new stars, or they grow by merging with smaller galaxies.

Astronomers hope to use Webb to find out whether different mechanisms were at work in the early universe.

“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 Dr 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.”

In this galaxy, the star formation activity is strongly rising toward the outskirts, as the star formation spreads out - a type of growth predicted with theoretical models, but now observed in the early universe for the first time, thanks to Webb.