Astronomers have reconstructed the full 12-billion-year growth history of a giant spiral galaxy by reading chemical fingerprints embedded in its gas; the first time this technique has been applied to a galaxy beyond the Milky Way.
The study, led by the Center for Astrophysics at Harvard and Smithsonian and published in Nature Astronomy, focused on NGC 1365, a large barred spiral galaxy visible face-on from Earth. Its orientation gave researchers the resolution needed to isolate individual star-forming clouds across the entire disk.
The key marker was oxygen. When young, hot stars ignite, their ultraviolet light excites surrounding gas, causing each element to emit narrow lines of light at specific wavelengths. Oxygen is particularly useful because its distribution across a galaxy reflects billions of years of stellar explosions, gas flows and collisions with other galaxies.
How a Small Galaxy Became a Giant
By mapping how oxygen concentrations vary across NGC 1365 and comparing those patterns against simulations of roughly 20,000 galaxies in the Illustris Project, the team traced a clear developmental sequence. The galaxy’s central region formed early and accumulated heavy elements quickly. The outer arms grew far more gradually, fed by repeated mergers with smaller dwarf galaxies over the last few billion years.
The picture that emerges is of a galaxy that began small and built itself up through a long series of collisions; a process the simulations had predicted but never before confirmed in a galaxy outside our own.
“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. “This study shows that the astronomical processes we model on computers are shaping galaxies like NGC 1365 over billions of years.”
A New Branch of Astronomy Takes Shape
Lead author Lisa Kewley, director of the Center for Astrophysics, described the method as the foundation of an entirely new field: extragalactic archaeology. The goal is not purely academic. NGC 1365 shares structural similarities with the Milky Way, and understanding how it formed could shed light on our own galaxy’s origins, including the oxygen in Earth’s atmosphere.
“We want to understand how we got here,” Kewley said. “How did our own Milky Way form, and how did we end up breathing the oxygen that we’re breathing right now?”
The team used data from the TYPHOON survey on the Irénée du Pont telescope at Las Campanas Observatory in Chile. Kewley noted the project was built equally on observation and theory, and that the collaboration between the two disciplines will likely reshape how astronomers approach galaxy formation research going forward.
Published by Kerry Harrison
Kerry’s been writing professionally for over 14 years, after graduating with a First Class Honours Degree in Multimedia Journalism from Canterbury Christ Church University. She joined Orbital Today in 2022. She covers everything from UK launch updates to how the wider space ecosystem is evolving. She enjoys digging into the detail and explaining complex topics in a way that feels straightforward. Before writing about space, Kerry spent years working with cybersecurity companies. She’s written a lot about threat intelligence, data protection, and how cyber and space are increasingly overlapping, whether that’s satellite security or national defence. With a strong background in tech writing, she’s used to making tricky, technical subjects more approachable. That mix of innovation, complexity, and real-world impact is what keeps her interested in the space sector.