Postdoc Spotlight: Yusuke Fujimoto explores our galactic history

Postdoc Spotlight: Yusuke Fujimoto explores our galactic history
Wednesday, March 24, 2021 

For most people, the question “Why are we here?” summons philosophical thoughts about the meaning of life and existential purpose. However postdoctoral fellow Yusuke Fujimoto takes that question very literally.

By simulating the vast spiraling structure of the Milky Way, Fujimoto aims to piece together where exactly we are in the galaxy, how we got here, and how that environment impacted (and impacts) our Solar System’s development. 

In this postdoc spotlight, Fujimoto explains why this work is important, how he uses mathematical models to account for 4.6 billion years of galactic evolution, and gives advice to current graduate students.

What is your area of research, and why do you think it is important?

Where do we come from? Where are we? Where are we going?

These are fundamental questions we human beings have had since time began, and they are the questions I hope to answer with my research. I particularly focus on our Solar System's movement and trajectory for the past 4.6 billion years within the Milky Way galaxy. 

To do that, I try to answer the following questions: Where and in what environment did our Solar system form? How have we wandered in the galactic disc during those 4.6 billion years? Where and in what environment are we now? To answer these big questions, I use numerical galaxy simulations.

What are the larger implications of your work?

Although the scientific objectives I mentioned are fascinating enough, my work can also significantly impact planet formation research. 

The formation mechanism of the Solar System and exoplanetary systems that may harbor life is one of the hottest topics in astronomy. Scientists investigate the early Solar System's birth environment 4.6 billion years ago using meteorites, which contain the oldest solids formed in the Solar protoplanetary disc. Materials from near-Earth asteroids by recent sample-return projects such as JAXA's Hayabusa-2 and NASA's OSIRIS-REx will give us even more information about the Solar birth environment and the formation history of the Solar system. 

At the same time, astronomers are discovering  many nearby exoplanets. In particular, after observing the protoplanetary disc of the HL Tauri young star by the ALMA radio telescope in 2015, many observational and theoretical works have focused on understanding present-day planet formation in our own Solar neighborhood. By combining results from those broad research fields, astronomers are trying to build a general theory of planet formation in our Universe. 

One challenge to these theories is the 4.6 billion-year gap between Solar System formation and the present-day planet formation. The Solar birth environment could be quite different from the current local environment in the galactic disc. Surprisingly, very few people notice this issue. 

My research about the Solar system's trajectory for 4.6 billion years within the Galaxy will be able to fill the gap.

How did you choose this field of study? What inspired you?

When I was a kid, I liked to read astronomy books. I remember that I was very impressed by the beautiful photos of galaxies that the Hubble Space Telescope took. That is why I choose galactic astronomy for my field of study. 

I am also interested in how galaxies move and change their structures (for example, spiral arms and bright star-forming regions).  However, it is almost impossible to directly observe galaxies' movement in real-time because their evolution timescales are hundreds of million years. 

That is why I am doing numerical galaxy simulations rather than observations; I am creating my Universe on a supercomputer and simulating galaxies however I like.

Can you summarize a recent publication (or project you're working on) and why it is important?

Yusuke Fujimoto creatures numerical simulations of the Milky Way galaxy in order to investigate the origins of our Solar System and how its environment may have changed over time. Above you see the results of these simulations.

I published a paper last year about the formation and evolution of the local interstellar environments around the Solar vicinity in the present-day Galaxy. 

By simulating the Milky-Way-like galaxy, I investigated the location of stars whose galactic environments look like Earth’s. I found that such stars may be uncommon but not exceptionally rare. These stars tend to be located near the edges of spiral arms and lie inside superbubbles created by multiple generations of massive star formation in the arm. 

It isn't easy to understand where exactly our Solar System is in the Galaxy because we reside in the galactic disc and cannot see the disc in face-on view. A galaxy simulation like this is a vital tool that can overcome this problem.  

Ultimately, this work has deepened our understanding of our Solar System’s location in the galactic disc. 

Formation and evolution of the local interstellar environment: combined constraints from nucleosynthetic and X-ray data, Fujimoto, Krumholz, Inutsuka, Boss and Nittler 2020, MNRAS 498, 5532

What makes those spiral arms so special?  Would you expect to see other Solar Systems like ours in similar places throughout our galaxy? 

These are great questions. Those spiral arms are not special, and we expect to see other interstellar environments similar to ours in other spiral arms in the Galaxy, in terms of superbubbles created by massive star clusters. 

The galactic spiral arm is where the interstellar gas gathers and forms stars (see the left and middle panels in the figure), which is why we observe shining grand-design arms in nearby spiral galaxies like our Milky Way. The massive stars in the spiral arms blow stellar winds and explode as supernovae at the end of their life, ejecting a large amount of material, including radioactive isotope 26Al (the right panel in the figure). They create many hot-gas bubbles along the spirals. In the paper, I found that the current Solar system might be close to such bubbles and that the probability that stars are traversing such environments is not exceptionally low.

How has your background influenced your research?

It is an interesting but tricky question.

I don't know, honestly. I always try to choose my research topic based on just its scientific significance. I always keep in mind that research should follow an appropriate procedure so that everyone can understand regardless of background.

Indeed, my background may influence my research, but I have never been aware of it.  

What else has influenced your thinking as a scientist?

All my collaborators I have ever worked with. Everyone has a slightly different attitude to research and philosophy as a scientist. I always learn a lot every time I work with them. That is why I prefer collaborative work rather than going solo.

When you're not actively researching, what hobbies or activities do you enjoy in your spare time?

I go jogging every day. It's my morning routine. And I occasionally watch Anime on Netflix. It's perfect for a change of pace!

Why did you choose Carnegie's Earth and Planets Laboratory?

Yusuke Fujimoto poses for a photo at the Earth and Planets Laboratory Holiday Party.

Carnegie EPL has many experts on planetary science, cosmochemistry, and geophysics, all of which are very far from my galactic astronomy research field. The interdisciplinary atmosphere at this institution lets me broaden my horizons! 

Do you have any advice for current graduate students?

Go abroad to study! It will be an excellent opportunity to learn about different peoples and cultures. 

Although it is not exactly advice, I would like to share one of my favorite quotes: “Be ambitious.” These were the parting words that American professor William S. Clark gave to his Japanese students when he was the president of Sapporo Agricultural College (now Hokkaido University, where I got my Ph.D.) in the late 1800s. This has become a nationally known motto in Japan and it is something that I think about often in my work. 

Find more about Yusuke Fujimoto’s work on his personal website.