By reading the chemical “fingerprints” of a distant galaxy, astronomers reconstructed its 12-billion-year evolution. This new method could reveal how galaxies—including our own—were built over cosmic time.
A team of astronomers led by the Center for Astrophysics | Harvard and Smithsonian has, for the first time, applied galactic archaeology to uncover the history of a galaxy beyond the Milky Way. This approach studies the chemical signatures left behind in space to piece together how galaxies form and evolve.
The findings, published today (March 23) in Nature Astronomy, introduce a powerful new method for reconstructing the life stories of distant galaxies. The work also helps establish a new research area known as “extragalactic archaeology.”
“This is the first time that a chemical archaeology method has been used with such fine detail outside our own galaxy,” says Lisa Kewley, lead author, Harvard professor, and director of the Center for Astrophysics. “We want to understand how we got here. How did our own Milky Way form, and how did we end up breathing the oxygen that we’re breathing right now?”
Mapping a Galaxy Using Chemical Fingerprints
To carry out the study, researchers used data from the TYPHOON survey, collected with the Irénée du Pont telescope at Las Campanas Observatory. They focused on NGC 1365, a nearby spiral galaxy that appears face-on from Earth, making it easier to observe in detail. This allowed the team to isolate and analyze individual regions where new stars are forming.
Young, hot stars emit strong ultraviolet light, which energizes nearby gas. According to Kewley, this process causes elements such as oxygen to produce distinct, narrow lines of light. By studying these patterns, scientists can determine how elements are distributed across the galaxy.
Astronomers have long known that galaxy centers tend to contain higher concentrations of heavy elements like oxygen, while outer regions have less. These patterns are shaped by several processes, including when and where stars form and explode as supernovae, how gas moves in and out of the galaxy, and past interactions with other galaxies.
Reconstructing 12 Billion Years of Galaxy Evolution
By tracking how oxygen levels vary across NGC 1365 and comparing those observations with advanced simulations from the Illustris Project, the team was able to reconstruct the galaxy’s growth over 12 billion years. These simulations model key processes such as gas movement, star formation, black hole activity, and chemical changes from shortly after the Big Bang to today.
Out of roughly 20,000 simulated galaxies, the researchers identified one that closely matched NGC 1365. This comparison allowed them to infer how the galaxy likely formed and evolved through mergers and gradual growth.
Their analysis suggests that the galaxy’s central region formed early and quickly became rich in oxygen. In contrast, the outer regions developed over billions of years through repeated collisions with smaller dwarf galaxies. The outer spiral arms appear to have formed more recently and were likely built up by incoming gas and stars from these mergers.
“It’s very exciting to see our simulations matched so closely by data from another galaxy,” said Lars Hernquist, Mallinckrodt Professor of Astrophysics at Harvard and a CfA astronomer. “This study shows that the astronomical processes we model on computers are shaping galaxies like NGC 1365 over billions of years.”
A New Tool for Understanding Galaxies
Overall, the findings show that NGC 1365 began as a relatively small system and gradually grew into a massive spiral galaxy through multiple mergers with smaller neighbors.
Kewley says this work demonstrates how chemical signatures in a galaxy’s gas can reveal its past, establishing extragalactic archaeology as a valuable new tool in astronomy.
“This study shows really well how you can produce observations to be directly aided by theory,” she said. “I think it’s also going to impact how we work together as theorists and observers, because this project was 50 percent theory and 50 percent observations, and you couldn’t do one without the other. You need both to come to these conclusions.”
What This Means for the Milky Way
Studying galaxies like NGC 1365, which shares similarities with the Milky Way, can help scientists better understand our own galaxy’s origins and whether its history is typical or unusual.
“Do all spiral galaxies form in a similar way?” asked Kewley. “Are there differences between their formation? Where is their oxygen distributed now? Is our Milky Way different or unique in any way? Those are the questions we want to answer.”
Reference: “The assembly history of NGC 1365 through chemical archaeology” by Lisa J. Kewley, Kathryn Grasha, Alex Garcia, Paul Torrey, Jeff Rich, Z. S. Hemler, Qian-Hui Chen, Peixin Zhu, Mark Seibert, Lars Hernquist and Barry Madore, 23 March 2026, Nature Astronomy.
DOI: 10.1038/s41550-026-02808-7
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