Astronomers have produced a detailed map of the Milky Way's magnetic field, revealing a surprising diagonal reversal in the Sagittarius Arm. The findings, based on new radio telescope data, help explain how this invisible force structures the galaxy. Led by scientists at the University of Calgary, the studies were published this month in The Astrophysical Journal.
Deep within the Milky Way, the galaxy's magnetic field plays a crucial role in maintaining its structure against gravitational collapse. Dr. Jo-Anne Brown, a professor in the Department of Physics and Astronomy at the University of Calgary, emphasized its importance: "Without a magnetic field, the galaxy would collapse in on itself due to gravity." She added, "We need to know what the magnetic field of the galaxy looks like now, so we can create accurate models that predict how it will evolve."
To map this field, researchers utilized data from a radio telescope at the Dominion Radio Astrophysical Observatory in British Columbia, operated by the National Research Council Canada. The observations scanned the northern sky across multiple radio frequencies as part of the Global Magneto-Ionic Medium Survey (GMIMS). Dr. Anna Ordog, lead author of one study, noted, "The broad coverage really lets you get at the details about the magnetic field structure."
The team measured Faraday rotation, a phenomenon where radio waves shift as they pass through regions of electrons and magnetic fields. Rebecca Booth, a PhD candidate working with Brown and lead author of the second study, described it: "You can think of it like refraction. A straw in a glass of water looks bent because of how light interacts with matter. Faraday rotation is a similar concept, but it's electrons and magnetic fields in space interacting with radio waves."
A key discovery focused on the Sagittarius Arm, where the magnetic field flows counterclockwise, opposite to the galaxy's overall clockwise direction. Brown recalled the breakthrough: "If you could look at the galaxy from above, the overall magnetic field is going clockwise. But, in the Sagittarius Arm, it's going counterclockwise. We didn't understand how the transition occurred. Then one day, Anna brought in some data, and I went, 'O.M.G., the reversal's diagonal!'"
Booth developed a three-dimensional model for this reversal, explaining its appearance from Earth. "My work presents a new three-dimensional model for the magnetic field reversal. From Earth, this would appear as the diagonal that we observe in the data," she explained. The complete dataset and model, published in The Astrophysical Journal and The Astrophysical Journal Supplement Series, provide tools for global astronomers to study the field's evolution.
These studies build on international collaboration, offering unprecedented insights into the Milky Way's hidden architecture.