Detailed images of one of the first galaxies show that growth in the early universe was much faster than previously thought. Astronomers are currently experiencing a productive period of discovery, exploring many mysteries of the early universe.

The successful launch of the James Webb Space Telescope (JWST), the successor to NASA’s Hubble Space Telescope, has pushed the boundaries of what we can see.

The observations now cover the first 500 million years after the Big Bang, when the universe was less than five percent of its current age. For humans, this time the universe will definitely move into the toddler stage. But the galaxies we observe are certainly not childish. New observations help rewrite our understanding of galaxy formation and evolution by revealing that galaxies are larger and more mature than expected for such early times.

Our international research team recently conducted unprecedentedly detailed observations of Gz9p3, one of the oldest known galaxies. Nature Astronomy.

It takes its name from the Glass collaboration (the name of our international research group) and the fact that the galaxy has a z=9.3 redshift; where redshift is a way of describing the distance to the object – hence G and z9p3. Just a few years ago, Gz9p3 appeared as a single point of light from inside the Hubble Space Telescope. However, with the help of the James Webb Space Telescope, we were able to observe this object 510 million years after the Big Bang, that is, approximately 13 billion years ago.

We found that Gz9p3 is much larger and more mature than expected for such a young universe that already contains several billion stars. Today, it is the most massive object confirmed since then and is estimated to be 10 times more massive than other galaxies found in the early universe.

Taken together, these results suggest that for a galaxy to reach this size, stars must evolve much faster and more efficiently than we initially thought.

The most distant galaxy merger in the early universe

Not only is this Gz9p3 massive, but its complex shape immediately identifies it as one of the oldest galaxy mergers ever observed.

The JWST image of the galaxy shows a morphology typically associated with two interacting galaxies. And the merger is not over because we still see two components.

When two large objects collide like this, they actually eject some matter in the process. So this expelled material shows that what we are observing is one of the most distant mergers ever seen. Our work was then taken further to identify the star population that forms merging galaxies. Using JWST, we were able to study the spectrum of the galaxy by splitting the light, much like a prism splits white light into a rainbow.

Most studies of these very distant objects using only images show only very young stars, because young stars are brighter and therefore their light dominates the image data.

For example, a bright population a few million years younger, caused by galaxy mergers, dwarfs a population older than 100 million years. Using spectroscopy techniques, we can make such detailed observations that two populations can be distinguished from each other.

New models of the early universe

Such a mature and old population was not expected, given the way early stars formed to become sufficiently old by this cosmic time. Spectroscopy is so detailed that we can see subtle features of ancient stars, which tells us there is more to it than you think.

Certain elements detected in the spectrum (including silicon, carbon, and iron) suggest that this older population must have existed to enrich the galaxy with abundant chemicals. What is surprising is not only the size of the galaxies, but also the speed at which they reach such a chemically mature state.

These observations suggest a rapid and efficient accretion of stars and metals immediately after the Big Bang and that this is associated with ongoing galaxy mergers, and suggest that massive galaxies consisting of several billion stars existed earlier than expected.