Billions of years ago, a young spiral galaxy began to grow in a crowded part of the universe. It pulled in gas and small companion galaxies, slowly building up the bright central region and sweeping spiral arms we see today.
In a new study published in March 2026, my colleagues and I used this galaxy’s chemical fingerprints to reconstruct its life story in detail.
Astronomers want to know how spiral galaxies like our own Milky Way came to be, as these galaxies can give us hints about how the elements we rely on, such as oxygen, were created and spread through space over time.
Space archaeology
Like archaeologists sometimes use slices of earth to to turn back the clock and study the Earth’s natural history, we used slices of data of the galaxy’s chemical makeup from different periods in time, alongside sophisticated galaxy evolution models. Together, the data helped us piece together how it formed and grew over 12 billion years.
The galaxy, called NGC 1365, lies relatively nearby, in cosmic terms, and is tilted so we see its spiral disk face-on. Using the du Pont telescope at Las Campanas Observatory in Chile, we mapped oxygen across thousands of star-forming gas clouds.
We then searched through simulations of about 20,000 model galaxies and found one that very closely matched NGC 1365. We looked at a host of factors while matching up the simulations, including the abundance of heavy elements, including oxygen. We used the model to rewind the history of the galaxy and predict how it likely grew over time and merged with other galaxies.
Looking for heavy elements
Heavy elements are forged in stars and released in powerful supernova explosions within galaxies. Over time, this process builds up a traceable record that scientists can look for in the gas – like how archaeologists look for certain key elements in layers of soil.
Research has shown that the center of a galaxy usually ends up richer in heavy elements, while the outer regions have less. That pattern carries clues about when stars formed, how gas flowed in and out, and how often the galaxy collided and merged with others.
For the galaxy NGC 1365, we found that its central region likely formed early in its lifespan and quickly became rich in oxygen. Its outer disk, however, grew more slowly. Over billions of years, the galaxy probably collided with smaller dwarf galaxies, which brought in fresh gas and stars and helped build up the outer spiral arms. A lot of the gas now in the edges of the spiral arms likely arrived relatively late in the galaxy’s life.
Our work is some of the first to use such a detailed “chemical archaeology” technique outside our own Milky Way galaxy. By tying new, super-fine resolution observations directly to state-of-the-art simulations, we’ve opened up a new way to study how distant galaxies assembled over cosmic time.
Unanswered questions
We can reconstruct a history for NGC 1365 using both our simulations and observational data. But some details remain uncertain. Different combinations of gas flows and mergers can sometimes leave similar chemical patterns. We also don’t know yet whether NGC 1365’s life story is typical for large spiral galaxies, or whether it is unusual in ways that aren’t clear to us yet.
A few key things we have yet to uncover include: Do most spiral galaxies build their centers early and their outer disks slowly, as NGC 1365 appears to have done? How much do galaxy mergers versus gas inflow contribute to a galaxy’s growth? And, perhaps most interestingly, how does the history of NGC 1365 compare to that of our own Milky Way?
