Shattering Galactic Beliefs: Astronomers Uncover Surprising Magnetic Field Structures in Milky Way

A team of astronomers including those from the University of Tokyo created the first-ever map of magnetic field structures within a spiral arm of our Milky Way galaxy. Earlier research provided only a broad overview of galactic magnetic fields. However, this novel study uncovers that the magnetic fields within the galaxy’s spiral arms deviate markedly from this broad overview, displaying a significant tilt from the galactic average. These discoveries indicate that magnetic fields have a substantial influence on regions where stars are formed, implying their role in the formation of our solar system.

It might come as a surprise to some that magnetic fields can exist on scales larger than a planet. Most of our daily experience with magnetic fields involves either sticking things to our refrigerator, or perhaps using a compass to point north. The latter shows the existence of magnetic fields generated by our planet. Our sun also creates a vast magnetic field, and this can affect phenomena like solar flares. But magnetic fields that span the galaxy are almost too large to comprehend, and yet they likely have a role in the formation of stars and planets.

New Insights into the Milky Way’s Magnetic Structure

“Until now, all observations of magnetic fields within the Milky Way led to a very limited model that was uniform all over and largely matched the disc shape of the galaxy itself,” said Assistant Professor Yasuo Doi from the Department of Earth Science and Astronomy. “Thanks in part to telescope facilities at Hiroshima University capable of measuring polarized light to help us ascertain magnetic signatures, and the Gaia satellite launched by the European Space Agency in 2013, which specialized in measuring the distances to stars, we are able to build a better model with finer details in three dimensions. We focused on a specific area, the Sagittarius arm of our spiral galaxy (we are in the neighboring Orion arm), and found the dominant magnetic field there breaks away from the plane of the galaxy significantly.”

Rethinking Galactic Magnetic Field Models

Previous models and observations could only imagine a smooth and largely homogeneous magnetic field in our galaxy; whereas the new data show that although magnetic field lines in the spiral arms do roughly align with the galaxy at large, at small scales the lines are actually spread out across a range of distances due to various astrophysical phenomena such as supernovae and stellar winds.

The galactic magnetic fields are also incredibly weak, around 100,000 times weaker than Earth’s own magnetic field. Despite this, however, over long time spans, gas and dust in interstellar space are accelerated by these fields which explains the presence of some stellar nurseries — star-forming regions — that cannot be explained by gravity alone. This finding implies further mapping of the magnetic fields within our galaxy could help better explain the nature and evolution of the Milky Way and other galaxies too.

Future Research and Implications

“I am personally intrigued by the foundational process of star formation, pivotal to the creation of life, including ourselves, and I aim to grasp this phenomenon in its entirety with time,” said Doi. “We aim to further our observations and build better models of galactic magnetic field structures. This endeavor aims to provide observational insights into the accumulation of gas fueling active star formation within our galaxy and its historical development.”

Reference: “Tomographic Imaging of the Sagittarius Spiral Arm’s Magnetic Field Structure” by Yasuo Doi, Kengo Nakamura, Koji S. Kawabata, Masafumi Matsumura, Hiroshi Akitaya, Simon Coudé, Claudia V. Rodrigues, Jungmi Kwon, Motohide Tamura, Mehrnoosh Tahani, Antonio Mario Magalhães, Reinaldo Santos-Lima, Yenifer Angarita, José Versteeg, Marijke Haverkorn, Tetsuo Hasegawa, Sarah Sadavoy, Doris Arzoumanian and Pierre Bastien, 11 January 2024, The Astrophysical Journal.
DOI: 10.3847/1538-4357/ad0fe2

This research has been supported by JSPS KAKENHI grants 25247016, 18H01250, 18H03720, 20K03276, and 20K04013. M.Tahani is supported by the Banting Fellowship (Natural Sciences and Engineering Research Council Canada) hosted at Stanford University and the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) Fellowship. CVR thanks the Brazilian Conselho Nacional de Desenvolvimento Científico e Tecnológico – CNPq (Proc: 310930/2021-9). AMM’s work and optical/NIR polarimetry at IAG have been supported over the years by several grants from the São Paulo state funding agency FAPESP, especially 01/12589-1 and 10/19694-4. AMM has also been partially supported by the Brazilian agency CNPq (grant 310506/2015-8).

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