An artist’s impression shows the giant spiral galaxy NGC 1365 as it collides and merges with a smaller companion galaxy, stirring up star formation and redistributing gas and heavy elements. Using a new ‘space archaeology’ technique that reads the chemical fingerprints in the galaxy’s gas, astronomers have reconstructed how NGC 1365 grew over 12 billion years. Credit: Melissa Weiss/ CfA.

Astronomers used galactic archaeology techniques to trace the chemical ‘fossil record’ of a galaxy outside the Milky Way.

A team of astronomers led by the Center for Astrophysics | Harvard and Smithsonian have for the first time used galactic archaeology, the study of detailed chemical fingerprints in deep space, to trace the history of a galaxy outside the Milky Way.

The study demonstrates a new way to reconstruct the evolution of distant galaxies, and opens up a new field of astronomy, called ‘extragalactic archaeology’. 

Lead author Lisa Kewley, Harvard professor, and director of the Center for Astrophysics, said: “This is the first time that a chemical archaeology method has been used with such fine detail outside our own galaxy.

“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?” 

Six views of the spiral galaxy NGC 1365, as extracted from its spectro-photometric data cube, generated by the TYPHOON survey. On the far left is a broadband image of the galaxy balancing B (blue), V (visual) and R (red) continuum images to approximate what the human eye would see. The next image is a narrow-band image extracted from the TYPHOON data cube centred on the H alpha line of ionized hydrogen. Individual HII regions, powered by hot high-luminosity OB stars, are clearly seen outlining the two massive spiral arms. The next three images are slices centred on other diagnostic emission lines (Nitrogen, Sulfur, and a composite of all three diagnostic emission lines). The final panel shows the color-coded velocity field of NGC 1365. Credit: B Madore, The Observatories, Carnegie Institution for Science.

Resolution

Using data from the TYPHOON survey on the Irénée du Pont telescope at the Las Campanas Observatory, the scientists examined the nearby spiral galaxy NGC 1365, whose wide disc shape is oriented so we can see it face-on from Earth.

They achieved resolution sharp enough to separate and study individual star-forming clouds in the galaxy.

When they’re young, hot stars shine brightly in the ultraviolet, and that intense light can excite nearby gases, Kewley explained.

Each element, such as oxygen, in the gas then produces bright, narrow lines of light.

Astronomers know that the centres of galaxies usually have more heavy elements, including oxygen, while the outer parts have less.

The oxygen pattern is shaped by several factors, including where and when stars formed and exploded as supernovae, how gas has flowed in or out of the galaxy, and past mergers with other galaxies. 

By measuring how the oxygen patterns change across a galaxy and comparing with state-of-the-art galaxy simulations in the Illustris Project, the astronomers traced how the galaxy grew and merged with other galaxies over 12 billion years of cosmic time.

The simulations track the motion of gas, star formation, black holes, and chemical evolution in galaxies from shortly after the Big Bang to the present day.

20,000 galaxies 

The astronomers searched through simulations of about 20,000 galaxies and found one that closely matched NGC 1365’s observed properties, from which they inferred the galaxy’s likely merger and growth history.

The astronomers found that NGC 1365’s central region formed early in the galaxy’s history and developed a large amount of oxygen.

The gas further out built up over 12 billion years through collisions with smaller dwarf galaxies.

The gas in the outer spiral arms of the galaxy probably formed relatively late, over the last few billion years, and was also fed by gas and stars from merging dwarf galaxies.

Lars Hernquist, Mallinckrodt Professor of Astrophysics at Harvard and a CfA astronomer, said: “It’s very exciting to see our simulations matched so closely by data from another galaxy.

“This study shows that the astronomical processes we model on computers are shaping galaxies like NGC 1365 over billions of years.”

Powerful new approach

Overall, the study shows NGC 1365 began as a small galaxy and slowly grew into a giant spiral via multiple mergers with smaller dwarf galaxies. 

The astronomers establish extragalactic archaeology as a powerful new approach and tool that demonstrates that chemical fingerprints in a galaxy’s gas can reveal its history, said Kewley. 

She stated: “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% theory and 50% observations, and you couldn’t do one without the other.

“You need both to come to these conclusions.” 

By studying galaxies like NGC 1365, which bears similarities to the Milky Way, astronomers can gain insight into how typical or unusual our own galaxy may be and the different pathways galaxies can take to reach their current states 

Kewley said: “Do all spiral galaxies form in a similar way? 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 study is published in the journal Nature Astronomy.