A team of astronomers has for the first time traced the full evolutionary history of a galaxy beyond the Milky Way by analyzing its chemical composition. Using oxygen mappings of the spiral galaxy NGC 1365 and comparing them to simulations, researchers detailed its growth over 12 billion years. The findings, published in Nature Astronomy, reveal an early-forming core and outer regions built through mergers.
Astronomers led by the Center for Astrophysics at Harvard and Smithsonian have pioneered a technique called extragalactic archaeology to uncover the past of NGC 1365, a nearby spiral galaxy oriented face-on to Earth. By mapping oxygen abundance across the galaxy with data from the TYPHOON survey, collected via the Irénée du Pont telescope at Las Campanas Observatory, the team identified chemical signatures left by star formation, supernovae, and gas flows over cosmic time. Young, hot stars in active regions ionize gas, producing measurable emission lines from elements like oxygen, which vary from the metal-rich core to the poorer outskirts. Lisa Kewley, Harvard professor and director of the Center for Astrophysics, who led the study, said: 'This is the first time that a chemical archaeology method has been used with such fine detail outside our own galaxy. 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?' Researchers matched these observations against simulations from the Illustris Project, which models galaxy evolution from near the Big Bang. Among 20,000 simulated galaxies, one closely resembled NGC 1365, indicating its central bulge formed rapidly and became oxygen-enriched early on. The outer disk and spiral arms, however, assembled gradually through repeated mergers with dwarf galaxies, bringing in fresh gas and stars. Lars Hernquist, Mallinckrodt Professor of Astrophysics at Harvard, noted: 'It's very exciting to see our simulations matched so closely by data from another galaxy. This study shows that the astronomical processes we model on computers are shaping galaxies like NGC 1365 over billions of years.' Kewley emphasized the synergy: 'This study shows really well how you can produce observations to be directly aided by theory... You need both to come to these conclusions.' NGC 1365's history offers insights into whether the Milky Way followed a typical path, with Kewley posing: 'Do all spiral galaxies form in a similar way?'