Astronomers have proposed for the first time that they can reconstruct the billion-year "life history" of a galaxy from just one observation. This new method is called "Extragalactic Archaeology". The research team uses artificial intelligence and the "fingerprints" of chemical elements within galaxies to write "biographies" for galaxies beyond the Milky Way that span the age scale of the universe.
Relevant research was recently published in Nature Astronomy. The author believes that this work is expected to become the basis of a new "field guide" for understanding how galaxies form, merge and evolve over the long cosmic time. To this end, the researchers started with a specific case: analyzing the small differences in the distribution of oxygen elements inside the spiral galaxy NGC 1365.
Generally speaking, the oxygen content is highest in the center of the galaxy and gradually decreases from the center outward, which is the typical distribution gradient of these heavier elements. But if there are abnormal fluctuations in this gradient, it may mean that the galaxy has experienced major "life events": such as the birth of new stars or black holes, supernova explosions of old stars, or collisions and mergers between galaxies.
The research team constructed about 20,000 evolution simulations of different types of galaxies, covering various "pains" that galaxies may experience during their growth, including star formation, black hole activity, gas movement, and the evolution of different chemical elements, etc., and set up a complete "life background" for these simulated galaxies. The team then used artificial intelligence to compare these simulated data with the actual observed NGC 1365, looking for scenarios that closely matched the chemical fingerprint of the galaxy, thereby inferring its evolution path.
"We can reconstruct the detailed growth history of a spiral galaxy based only on its current chemical fingerprint." Lisa Kewley, the first author of the paper and director of the Harvard-Smithsonian Center for Astrophysics, said in an interview with Refractor.
The team's analysis shows that the central region of NGC 1365 was formed in the early universe, almost dating back to the birth of the universe 13.7 billion years ago, and oxygen was produced at this stage. Over the next approximately 12 billion years, the gas in the outer reaches of the galaxy was continuously replenished through collisions and mergers with dwarf galaxies. The outer gas gradually accumulated and pushed the galaxy to grow. These merged dwarf galaxy stars also brought new sources of matter to the outer edges.
This is just the story of one galaxy, and the goal of "extragalactic archeology" is to reconstruct the "history of life" for the vast number of galaxies beyond the Milky Way. Researchers hope to find the "fossil record" of these galaxies in this way: Although galaxies do not have skeletons like life on Earth, the gas, dust, stars, dark matter structures, etc. inside them have left traces of their long evolution that can be traced.
In traditional research, astronomers usually rely on "red shift" to characterize the distance and age of galaxies - the more obvious the spectrum shifts toward the red end, it means that the galaxy is farther away and the earlier stage of the universe is observed. This is because the universe as a whole is expanding, and the earliest galaxies are moving away from each other at a faster rate.
Redshift is still an important tool in measuring the age of galaxies, but "extragalactic archeology" attempts to answer a different kind of question: how galaxies merged, exchanged gas and other materials over billions of years, and how these processes shaped them into what they are today. Fortunately, the combination of new technologies such as artificial intelligence, large-scale numerical simulations, and high-resolution telescopic observations has made this ambitious research path gradually feasible.
In Ciuli's view, NGC 1365 is an ideal case for conducting this type of "archaeological" research. "This study shows that fine imaging of oxygen within a galaxy, if systematically compared with thousands of simulated galaxies, can significantly narrow down its possible evolution paths," she said.
Of course, NGC 1365 is just one of tens of billions of galaxies in the universe. The researchers emphasized that the next step is to apply the same method to more types of galaxies and cover different merger sequences and evolutionary branches. If all goes well, they hope to eventually compile a "field guide to typical galaxies": just like seeing a bird on Earth, people can not only describe its current appearance characteristics, but also infer its growth environment and evolution process. Astronomers can also use the "chemical fingerprint" and structural "appearance" of the galaxy to infer when and how its stars and gas converged into what they are today.
As a research direction that is still in its infancy, Chiuly pointed out that "extragalactic archeology" still has many areas to be improved. "As we incorporate more chemical elements, more galaxy samples, and more sophisticated simulations, this method will become more powerful." She said, adding that the research team's long-term expectation is to use this new tool to better reveal the history of distant galaxies, not only to help humans understand the evolution of the entire universe, but also to give us a clearer understanding of the location and origin of our own galaxy in the universe.