Frontiers of Science

Fascinated by the shapes of galaxies

How are the spiral arms of a galaxy formed?


Assistant Professor, Institute of Astronomy

September 1, 2021


How can we understand the changes taking place in the universe?
Unraveling the mystery behind galaxies changing shape over time.

The shape of the universe is changing, but it’s difficult for humans to recognize because our time span is too short compared to that of the universe.

The shapes of galaxies, which number in the trillions, have also changed over time. Six billion years ago, irregular galaxies with amorphous star distributions accounted for the majority of galaxies in the universe, but today, spiral galaxies such as the Milky Way account for more than 70% of the total population.

How have spiral galaxies been formed during this period? Assistant Professor Fumi Egusa addresses this mystery in her research.

“If we were to spend 100 million years observing galaxies, we'd know straightaway, but that's impossible. We're trying to figure out why there are so many spiral galaxies now from our current observations alone.”

There are several models describing how spiral galaxies may be formed. For example, the density-wave model suggests that the overall shape of the spiral is retained even though individual stars are moving. In the dynamic spiral arm model, each arm is repeatedly created and destroyed, but the shape as a spiral galaxy is retained over the long term.

Research to date has suggested that spiral galaxy morphology cannot be explained by using just one model. Assistant Professor Egusa is therefore looking closely at spiral galaxies in an attempt to classify them according to the various models.

The target of her observations and analysis is the distribution of stars and gases that make up spiral galaxies.

“Theoretical calculations have shown that the density-wave model and the dynamic spiral arm model lead to different distributions of stars and gases. In the former, the locations in which stars and gases are distributed are different, but in the latter, stars and gases are essentially distributed in the same location. When we looked at one particular galaxy, we found that one arm was close to the density-wave model and the other was close to the dynamic spiral arm model. It's quite likely that a single galaxy can have different types of arms.”

For her observations and analysis, Egusa uses data from the ALMA Telescope in Chile, operated by the National Astronomical Observatory of Japan with an international collaboration. Researchers including Egusa use different types of data obtained from different telescopes around the world, depending on their scientific goals.

When she was in elementary school, Egusa was drawn to the beauty of space in an NHK TV science program. She entered the Department of Astronomy as she had hoped, and first came across the spiral galaxy M99 as a candidate research project in her fourth year. Then the laboratory head Professor Yoshiaki Sofue (currently professor emeritus) invited her to take it on as a project, as no-one else had. “I came to prefer spiral galaxies to theory.” she mused.

Her work on spiral galaxies has taken her to several research institutes across the globe. As someone who has experienced many different astronomy research environments, Assistant Professor Egusa says that the Department of

Astronomy “covers a wide spectrum of research, with several areas of interest in each laboratory.” Interactions with members of other laboratories are frequent, and joint research is possible across different fields.

Nevertheless, the focus of her research remains squarely fixed on spiral galaxies.

“I’m delighted when we obtain the results we expect, but it's also fascinating to see unexpected findings that raise new questions. Going forward, I’m looking forward to continuing my observations and research into spiral galaxies.

Interview and text: Jiro Urushihara
Editing​: Masatsugu Kayahara
Photography: Junichi Kaizuka

Originally published in The School of Science Brochure 2020

Assistant Professor, Institute of Astronomy
Graduated from the Department of Astronomy, Graduate School of Science, The University of Tokyo in 2008, where she was awarded a Ph.D. (Science). After working as a researcher at the California Institute of Technology, the Institute of Space and Astronautical Science (JAXA), and the National Astronomical Observatory of Japan (NAOJ), and as a specially appointed assistant professor at NAOJ, she assumed her current position in 2018.


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