For the first time, astronomers have decoded the chemical fingerprints of a distant spiral galaxy, revealing how it grew and evolved over billions of years, using what researchers liken to “extragalactic archaeology.”
The team included members of the Harvard-Smithsonian Center for Astrophysics, and their work was published in Nature Astronomy, examining the nearby spiral galaxy NGC 1365. Extragalactic archaeology is an emerging discipline in astronomy that enables researchers to understand the evolution of distant galaxies by comparing clues in their present makeup to trace their evolution.
The new research was published on March 23, 2026, in the journal Nature Astronomy.
Extragalactic Archaeology Explained
“This is the first time that a chemical archaeology method has been used with such fine detail outside our own galaxy,” said lead author Lisa Kewley, 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?”
The new technique involves collecting detailed spectroscopic data on distant galaxies to analyze their present composition. These observations are then compared to a massive catalog of computer simulations of galactic evolution—specifically, the Illustris Project in this case—to determine the best match for how a galaxy developed into its current state.
“In earth-based archaeology, we use the properties of present-day fossils to trace the history of life and plant forms on earth, with some fossils showing records of major events,” Kewley explained to The Debrief. “This is analogous to how we are using oxygen abundance to trace the history of a galaxy and the major events in its past.”
Data for Extragalactic Archaeology
Data for the project were collected using the Irénée du Pont Telescope at Las Campanas Observatory during the TYPHOON survey. NGC 1365 is favorably oriented, with its wide disk facing Earth, allowing the telescope to capture a clear, high-resolution view of the galaxy. This level of detail enabled researchers to study the galaxy’s star-forming regions individually.
“In detail, the Carnegie TYPHOON survey gives us a three-dimensional data cube of a galaxy like NGC 1365,” Kewley explained. “The data cube is like an image from a mobile phone, where each pixel gives a spectrum with emission lines from particular chemical elements. We see hydrogen, nitrogen, sulfur, and oxygen, and can make maps of these elements across the galaxy at much higher resolution than we have been able to in the past.”
The bright ultraviolet light produced by young, hot stars excites surrounding gas, which then emits distinct spectral lines that allow astronomers to determine its composition. Typically, heavier elements such as oxygen tend to concentrate toward a galaxy’s center. A variety of processes influence how oxygen is distributed, including star formation and death, gas flows, and galaxy mergers.
Extragalactic Archaeology Reveals the Past
“We searched through about 20,000 simulated galaxies and asked: which of these ended up with a similar mass, and looking and behaving, in a chemical sense, like the one we observed?” Kewley explained. “The closest match gives us a strong, physics-based guess at the kinds of mergers and gas flows that must have happened. There weren’t really any other close matches in the simulations, but we plan to follow up this work with more nearby galaxies and more simulations.”
According to the best-fit simulation, the galactic center formed early on and produced a significant amount of oxygen. Over the next 12 billion years, collisions with dwarf galaxies increased the amount of oxygen. The galaxy’s spiral arms were likely a late addition, fed by gas from various mergers occurring over the last few billion years.
The simulation indicates that NGC 1365 most likely had humble beginnings as a small galaxy, only growing into a giant spiral over many mergers. The researchers say that this is powerful evidence of what extragalactic archaeology can achieve.
“Our work is a first step into this new capability to use the oxygen record to infer the history of galaxies outside our own using these techniques,” Kewley concluded. “Next, we want to build that up into a catalog of galaxies with different shapes and sizes, galaxies that have had quiet lives and galaxies that have crashed into other galaxies by major collisions. The goal is to turn extragalactic archaeology into a tool so that for any galaxy we can ask not just ‘What does it look like now?’ but ‘How did it get this way?’”
The paper, “The Assembly History of NGC 1365 Through Chemical Archaeology,” appeared in Nature Astronomy on March 23, 2026.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.