Extragalactic Archaeology: Unveiling the 12-Billion-Year Chronicle of Spiral Galaxy NGC 1365

For the first time, astronomers have successfully applied the rigorous techniques of chemical archaeology to a galaxy far beyond our own Milky Way, ushering in a new scientific frontier known as "extragalactic archaeology." This landmark study, which appeared in the journal Nature Astronomy on March 23, 2026, reconstructed the 12-billion-year evolution of the spiral galaxy NGC 1365 by deciphering chemical "fingerprints" embedded in cosmic gas. The research was spearheaded by Lisa Kewley, the Director of the Center for Astrophysics | Harvard & Smithsonian (CfA), who emphasized that this represents the inaugural instance of such a comprehensive chemical analysis being conducted on an external galaxy.

The foundational data for this ambitious undertaking was gathered through the TYPHOON survey, utilizing the Irénée du Pont Telescope located at the Las Campanas Observatory. This equipment provided the high spatial resolution necessary to scrutinize individual star-forming regions within the distant system. By mapping oxygen distribution across more than 4,500 spatial pixels, or "spaxels," the team used the element as a primary tracer. Because oxygen is rapidly produced by massive stars and dispersed via supernova explosions, these oxygen maps act as a celestial palimpsest, where every major event—from stellar bursts to galactic mergers—leaves a distinct and readable signature.

The findings indicate that NGC 1365 began its life as a much smaller system, with its central core forming exceptionally early in cosmic history. This central region became enriched with oxygen between 11.9 and 12.5 billion years ago, primarily through the absorption of smaller dwarf galaxies. In contrast, the galaxy's outer spiral arms developed at a later stage, fueled by subsequent gas accretions. To interpret these intricate chemical signals, the researchers compared their empirical data with cosmological simulations from the Illustris project, specifically the TNG0053 model. This involved analyzing approximately 20,000 simulated galaxies, a massive computational effort developed with contributions from Volker Springel and Mark Vogelsberger.

Historically, the methodology of galactic archaeology was confined to the Milky Way to understand our own origins, making this expansion into deep space a monumental leap for astrophysics. The selection of NGC 1365, situated within the Fornax Cluster at a distance of roughly 18.1 megaparsecs, was strategic. Its nearly face-on orientation relative to Earth allowed for an unobstructed and high-resolution view of its galactic disk, greatly simplifying the data collection process. These results provide strong support for the prevailing cosmological model, which suggests that large spiral galaxies grow through a series of mergers with smaller neighboring systems.

Lars Hernquist, a professor of astrophysics at Harvard and an astronomer at the CfA, noted that the study vividly illustrates how simulated astronomical processes mirror the actual formation of galaxies over billions of years. Applying this archaeological lens to NGC 1365 establishes a vital framework for future comparative studies, helping scientists determine if the Milky Way’s developmental path is a standard trajectory or a unique anomaly among large spiral systems. The successful alignment of observational data with the Illustris simulations provides a high degree of confidence in the established timeline, confirming that while the center of NGC 1365 matured early, its outer structures were built up gradually over eons.