Research Byte

Published in the RSAA Lunations
Vol1 Issue71 1–30 April 2026

We’ve known for a long time that the interstellar medium (ISM) carries a detailed record of a galaxy’s past. However most of that work has focused on stars, especially in the Milky Way. What’s been missing is the ability to read that record directly in the gas in other galaxies. Using 3D integral field data from the TYPHOON survey, what we have done is map the oxygen distribution across the entire star-forming disk of NGC 1365 at scales close to individual star-forming regions.

NGC 1365 was chosen as an intriguing case because what we see is that the ISM isn’t chemically smooth: it was broken into three distinct zones, and each one points to a different stage in the galaxy’s growth. The inner region shows long-term enrichment driven by gas inflows and star formation, the main disk reflects early assembly and gradual build-up, and the outer disk appears to have formed later through accretion and minor mergers. In other words, the gas itself retains a memory of how the galaxy formed.

To try and understand what was going on, we compared directly with cosmological simulations from IllustrisTNG. We shockingly found a simulated galaxy that match what we observe today in NGC1365. We then used the history of the simulated galaxy to infer how NGC 1365 likely assembled—through a combination of early growth, ongoing star formation, and later mergers.

What’s exciting here is that we’re no longer limited to broad statistical trends or stellar archaeology alone. By combining detailed maps of the ISM with simulations, we can start to reconstruct the histories of individual galaxies in a much more direct way.

The next step is to do this for larger samples. That will let us compare how different galaxies build up over time and start to answer bigger questions, such as whether galaxies like the Milky Way follow a common pathway, or whether their histories are more diverse than we expect.

Kathryn Grasha

Image: The TYPHOON RGB image and gas-phase metallicity maps of NGC 1365 are compared with the face-on projections of the z = 0 distributions in TNG0053 of gas surface density Σgas and stellar surface density Σ⋆, gas-phase metallicity, and stellar metallicity.

Original paper: DOI 10.1038/s41550-026-02808-7 

The assembly history of NGC 1365 through chemical archaeology. Nature Astronomy. Lisa J. Kewley, Kathryn Grasha, Alex Garcia, Paul Torrey, Jeff Rich, S. Hemler, Qian-Hui Chen, Peixin Zhu, Mark Seibert, Lars Hernquist, Barry Madore.