Using a multiwavelength approach to determine star formation rates in spiral arms
Spiral galaxies, with their sweeping arms, have long been thought to be stellar nurseries. But new research led by UMass Amherst PhD student Bingqing Sun suggests these spiral arms may not trigger star formation – they may simply gather the raw material, gas and dust, needed to form stars. For Sun, tackling this question meant rethinking how star formation is measured, using a new approach that combines observations across the electromagnetic spectrum.
Spiral galaxies are among the most common galaxy types in our local universe, making up roughly 70% of those we observe. Their defining features, the spiral arms, are often traced by bright, young stars, suggesting a link to recent star formation. Astronomers have long debated the role these spiral arms play in that process. One possibility is that spiral arms actively trigger star formation by compressing gas and dust as it flows through dense regions. Alternatively, the arms may primarily act as material gatherers, funneling gas and dust into dense regions where stars would form regardless. Distinguishing between these scenarios has proven challenging.
To address this, Sun took a multiwavelength approach, analyzing six nearby spiral galaxies using a dataset spanning ultraviolet to near infrared wavelengths. Instead of relying on a single wavelength, an approach that can miss key aspects of star formation, they used spectral energy distribution (SED) fitting, which combines light across many wavelengths to estimate physical properties such as star formation rate. Sun further divided each galaxy into thousands of small regions, each about 3,000 to 5,000 light-years across, and compared properties between spiral arm and interarm regions.
Her results revealed no significant differences in how quickly new stars are forming relative to a galaxy’s existing stars, or in how efficiently gas is turned into stars, between spiral arm and interarm regions. Instead, the findings suggest that spiral arms primarily act as structures that gather gas and dust, rather than environments that actively trigger star formation.
Beyond this result, Sun notes that the ability to measure star formation rates and efficiencies using this multiwavelength approach provides valuable tools for studying distant, high-redshift galaxies, where direct measurements of gas content are often not possible. She also plans to expand her sample to include a wider range of galactic environments, such as mergers and interacting systems, to test which galactic environments may favor one scenario over the other.
For Sun, one of the most difficult aspects of this project was not the analysis itself, but rather communicating the results. “It’s easy to lose track of how much you’ve learned along the way,” she notes, highlighting the challenge of stepping back and presenting complex work in a clear and accessible way. At the same time, there were moments when it all paid off. After working tirelessly for months on the SED modeling, seeing the individual galaxy fits marked a turning point in the project – the first clear sign that the approach was working as intended.
Working on a project in such a highly debated area has given Sun a refreshingly selfless perspective on their research. When asked whether she favored one scenario over the other, Sun replied, “I don’t have a preference, I’m more curious: if we improve how we calculate star formation rates, where will that lead us?” This perspective reflects a broader scientific mindset: prioritizing a clearer understanding over confirming a particular hypothesis, a view shaped by countless hours of effort. Sun also emphasizes to young astronomers that it is okay to make mistakes. Mistakes, they explain, are not setbacks but an essential part of becoming a better scientist. Developing the willingness to struggle, adapt, and grow may be just as important as any technical skill.
About the author: Dan Kidwell is a senior undergraduate studying astronomy and physics at UMass Amherst. He currently works in high-energy astronomy, conducting simulated observations of stellar winds in the Galactic Center. Outside of his studies, he enjoys hiking, camping, and photographing the night sky.