July 29, 2025
Study measuring magnetic field near the center of the Milky Way helps to decode the precise astrophysical dynamics at the heart of our galaxy
The underlying physics governing the center of our galaxy (the Galactic Center), due to its chaotic and complex nature, has been difficult to observe, model, and predict. Studying the region’s interactions and the environment where they occur helps to unravel the mystery and lead to a better understanding of the center of our, and even other, galaxies.
The central region of the Milky Way, known as the Central Molecular Zone (CMZ), is a vast reservoir of interstellar gas and dust orbiting the center of the galaxy and an ideal place to study astrophysics in extreme environments. One particular site within the CMZ named Sagittarius C (Sgr C) is known for intriguing cloud and filamentary features, complex dynamical structure, and massive star formation.
To better understand this region of space, a team including Roy Zhao, a second-year PhD student in the Department of Physics and the Kavli Institute for Cosmological Physics, measured the magnetic field of Sgr C, publishing the results in the Astrophysical Journal.
Zhao, the first author on the study, currently works with Professor Emeritus Josh Frieman and Associate Professor Chihway Chang in the Astronomy and Astrophysics Department. He investigates the astrophysics of galaxies across the history of the universe, recently shifted to applying novel data-driven techniques to the study of cosmology.
This study originated when Zhao was a researcher at UCLA, in collaboration with Distinguished Professor Emeritus Mark Morris, PhD’75. Zhao presented his research at the American Astronomical Society’s 244th meeting and describes its goals, major findings, and next steps in the Q&A below.
How was the study conducted?
Sagittarius C is known to be a bright radio and infrared source. To better understand the status of this region, we studied the magnetic field there by observing the infrared light emitted by the interstellar dust grains peppered through the clouds. The dust grains are aligned by the magnetic field like compasses, so the infrared light they emit is polarized and can therefore be used to infer the orientation of the magnetic field. The observation was done by NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA), a Boeing 737 modified into a flying telescope, which was retired in 2022.
In our galaxy, this region constitutes the astrophysical “Rosetta Stone,” allowing us to piece together the interaction between three key components—dense clouds, star formation, and the relatively strong magnetic field—and what such interaction may cause (e.g. excessive star formation, strong stream of free electrons, etc.). Direct observation of the magnetic field plays an important role in deciphering this myth.
| Figures of the center of our galaxy and Sgr C complex provide astro-geographical context. In the zoom-in image of Sgr C, the cyan denotes warmer gas and dust, magenta is colder dust, and yellow depicts free electrons travelling at very high speeds. The stripes indicate the direction of the magnetic fields. This region comprises a pair of cold dust arms (magenta) merging from the top and bottom onto a warm region with hot electrons (the cyan and yellow clump in the middle, created by the presence of a presumed cluster of bright, massive stars). This hot region and the surrounding magnetic field together give rise to the bright yellow filament—a stream of bundled high-speed electrons. (Images made by David Chuss, Kaitlyn Karpovich, and Roy Zhao, using data from the Spitzer Space Telescope, ESA’s Herschel Space Telescope, and MeerKAT. Refer to Figure 1.) |
What are your key findings and their implications?
We found that the magnetic field is an excellent probe of the local environment. The magnetic field appears to wrap around an expanding central bubble of hot, ionized gas. This bubble has apparently been created by the powerful winds of a group of massive young stars, and those winds have compressed and accelerated the dense gas and its magnetic field into the observed expanding bubble.
Back in the 1980s, my advisor and second author of this work, Prof. Mark Morris, co-discovered the existence of radio-emitting filaments (the thin streams of high-speed electrons) in the dynamical center of the Milky Way called the Galactic Center. Since then, many hypotheses have been proposed regarding their formation. Our study found that the magnetic field geometry near the origin of the filament supports one of the leading hypotheses, known as magnetic field line reconnection, where two merging fields accelerate the nearby electrons to close to the speed of light and form these filaments. This theory can now be applied to many other radio filaments that span the Galactic Center region.
Meanwhile, we have also learned how star-formation regions, cold clouds, and hot, ionized regions interact with each other and how the shape of the magnetic field may have a profound impact in determining the outcome of the interaction. These lessons learned from this “Rosetta Stone” can be applied to other parts of our galaxy, so scientists can reconstruct the events taking place in such regions.
What are the next steps?
Our study was done with one wavelength of light (214 microns), which probes only colder clouds. As such, our results cannot speak fully to the magnetic field of hotter regions that are heated by sources such as stars. To have a complete picture of what’s happening in the region, we also require an understanding of the dynamics inside these warm regions. Therefore, a natural next step would be to carry out a similar survey at a different wavelength.
What led you to investigate this question?
While the Sgr C region is one of the most active star-forming sites in the Galactic Center, it has historically received far less attention, especially compared to Sgr A*, a nearby supermassive blackhole. However, Sgr C is of particular interest since it provides us with direct access to an ongoing interaction with three components: a bubble of ionized gas, cold clouds with active star-formation, and the radio filaments. The astrophysics waiting to be uncovered in this region is rich and will lead to a better understanding of the dynamics of our own galaxy.
Did anything about this work surprise you?
The most surprising part of this work was observing the amazing correspondence between our magnetic field measurement and other surveys conducted in the same region. For example, our results showed that the central HII region (the hot, ionized region) forms a blown-up bubble, where the magnetic field overlaps perfectly with a shell indicated by the [CII] emission line—a bright spectral line produced by singly ionized carbon atoms in clouds excited by the UV radiation from stars—measured by another SOFIA survey. Through some further investigation, we found a powerful star (called a Wolf-Rayet star) at the center of the bubble that may have created it. It was a very satisfying moment to see different pieces of the puzzle fit together seamlessly!
Citation: "SOFIA/HAWC+ Far-infrared Polarimetric Large Area CMZ Exploration Survey. V. The Magnetic Field Strength and Morphology in the Sagittarius C Complex." Roy J. Zhao et al 2025 ApJ 988 252.