Galactic Archaeology Reveals 12 Billion-Year History of NGC 1365 Galaxy

Astronomers have, for the first time, used a technique akin to “space archaeology” – analyzing the chemical composition of gas within galaxies – to reconstruct the 12-billion-year history of a giant spiral galaxy, NGC 1365. This breakthrough, detailed in a study published today in Nature Astronomy, offers a new method for understanding the evolution of galaxies beyond our own Milky Way and establishes a field the researchers are calling “extragalactic archaeology.” The work centers on tracing the origins of elements like oxygen within the galaxy to map its growth and interactions over cosmic time.

Unearthing Galactic History Through Chemical Fingerprints

The core of this research lies in the concept of galactic archaeology, which, until now, has largely been confined to studying the Milky Way. The method relies on analyzing the chemical “fingerprints” found in the gas between stars. Each element present in the gas emits a unique pattern of light when energized by young, hot stars. By meticulously measuring these patterns, astronomers can determine the abundance of different elements, like oxygen, and infer the processes that created them. “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?” explains Lisa Kewley, lead author of the study and director of the Center for Astrophysics | Harvard & Smithsonian.

The team focused on NGC 1365, a spiral galaxy approximately 60 million light-years away, chosen for its face-on orientation which allows for detailed observation of its structure. Data was collected using the TYPHOON survey and the Irénée du Pont telescope at the Las Campanas Observatory in Chile. This allowed the scientists to achieve a resolution high enough to distinguish individual star-forming regions within the galaxy. The intensity of light emitted by elements like oxygen reveals clues about the galaxy’s past.

Oxygen as a Cosmic Chronometer

Astronomers have long known that the centers of galaxies typically contain a higher concentration of heavy elements, including oxygen, compared to their outer regions. This distribution isn’t random; it’s shaped by a complex interplay of factors. Star formation rates, supernova explosions (which create and disperse heavy elements), the inflow and outflow of gas, and past mergers with smaller galaxies all contribute to the oxygen pattern. By mapping these patterns across NGC 1365, the researchers aimed to decipher the galaxy’s evolutionary timeline.

To interpret the observed oxygen distribution, the team turned to sophisticated computer simulations from the Illustris Project. These simulations model the complex physics governing galaxy formation and evolution, tracking the movement of gas, star birth, black hole activity, and chemical enrichment over billions of years. By comparing the observed oxygen patterns in NGC 1365 to the simulations, the astronomers could identify a simulated galaxy that closely matched the observed properties. This matching simulation then provided insights into the likely merger and growth history of NGC 1365.

A History of Mergers and Gradual Growth

The analysis revealed that the central region of NGC 1365 formed early in its history and quickly accumulated a significant amount of oxygen. The outer regions, however, built up more gradually over 12 billion years through a series of collisions and mergers with smaller dwarf galaxies. The gas in the spiral arms likely formed more recently, within the last few billion years, and was also enriched by material stripped from these merging dwarf galaxies. Essentially, NGC 1365 wasn’t born a giant; it grew into one by consuming smaller galaxies over cosmic timescales.

“It’s highly exciting to spot our simulations matched so closely by data from another galaxy,” said Lars Hernquist, a CfA astronomer and co-author of the study. “This study shows that the astronomical processes we model on computers are shaping galaxies like NGC 1365 over billions of years.” The success of this approach validates the underlying physics incorporated into these complex simulations.

Extragalactic Archaeology: A New Tool for Understanding Galaxy Formation

This study isn’t just about understanding NGC 1365; it establishes extragalactic archaeology as a powerful new tool for studying galaxy evolution. By analyzing the chemical fingerprints in a galaxy’s gas, astronomers can now reconstruct its history, even for galaxies billions of light-years away. This approach offers a complementary perspective to traditional methods that rely on observing the light emitted by stars.

Kewley emphasizes the collaborative nature of this work, stating, “This study shows really well how you can produce observations to be directly aided by theory… I think it’s also going to impact how we work together as theorists and observers, because this project was 50 percent theory and 50 percent observations, and you couldn’t do one without the other.”

Implications for the Milky Way and Beyond

By studying galaxies like NGC 1365, which share similarities with our own Milky Way, astronomers hope to gain a better understanding of our galaxy’s origins and whether it’s typical or unique. Questions remain about the specific pathways galaxies take to reach their current states. “Do all spiral galaxies form in a similar way?” Kewley asks. “Are there differences between their formation? Where is their oxygen distributed now? Is our Milky Way different or unique in any way? Those are the questions we want to answer.”

The team plans to apply this technique to a larger sample of galaxies, using data from future surveys and more powerful telescopes. This will allow them to build a more comprehensive picture of galaxy evolution and identify the common threads and unique characteristics that shape these vast cosmic structures. Further research will also focus on refining the simulations to better capture the complexities of galaxy formation and improve the accuracy of the reconstructed histories. The EurekAlert! release notes that this method could be applied to other galaxies as telescope technology improves.