Space archaeology is helping astronomers reconstruct the long history of NGC 1365, a prominent spiral galaxy that formed billions of years ago in a crowded patch of the universe. A study published in March 2026 used the galaxy’s chemical signatures to chart how it accumulated gas and smaller companions to build its bright center and sweeping arms.
Space archaeology and galaxy evolution
Borrowing the logic of archaeological stratigraphy, researchers analyzed slices of chemical data from different epochs and paired them with sophisticated galaxy evolution models to trace 12 billion years of growth. NGC 1365 sits relatively nearby in cosmic terms and is oriented nearly face-on, giving a clear view of its disk. Using the du Pont telescope at Las Campanas Observatory in Chile, the team mapped oxygen across thousands of star-forming gas clouds.
They compared those observations to simulations of about 20,000 model galaxies and identified one that closely matched NGC 1365. By aligning trends in heavy element abundance, including oxygen, the team used the best-fit model to rewind the galaxy’s past and infer how it expanded and merged with other systems over time.
Galaxies form when gravity and dark matter draw material toward their centers.
Looking for heavy elements
Heavy elements are forged inside stars and dispersed by supernovae, leaving a measurable chemical record in galactic gas, much like diagnostic markers in layers of soil. Prior research shows that galactic centers typically become enriched in heavy elements while outer regions remain comparatively sparse, encoding information about star formation, gas flows, and past collisions.
For NGC 1365, the team found its core likely formed early and became rapidly enriched in oxygen. The outer disk appears to have grown more gradually. Over billions of years, the galaxy probably encountered and merged with smaller dwarf galaxies, bringing in fresh gas and stars that helped build its outer spiral arms. Much of the gas now at the edges of those arms likely arrived relatively late in the galaxy’s life.
The study is among the first to apply such detailed chemical archaeology beyond the Milky Way. By linking high-resolution observations directly to state-of-the-art simulations, the researchers outlined a new pathway to study how distant galaxies assemble across cosmic time.
Unanswered questions
While the combined simulations and observations yield a coherent history for NGC 1365, uncertainties remain. Different patterns of gas inflow and mergers can produce similar chemical signatures. It is also unclear whether NGC 1365’s evolution is typical of large spiral galaxies or if it is unusual in ways that are not yet apparent.
Open questions include whether most spirals build their central regions early while their outer disks grow slowly, how the balance between mergers and gas inflow drives growth, and how NGC 1365’s trajectory compares with that of the Milky Way.