For hundreds of years, astronomers have studied the night sky to understand the forces that shape the universe. One of the most important yet invisible forces inside the Milky Way is its magnetic field. Now, researchers at the University of Calgary have produced one of the clearest views yet about its hidden structure.
“Without a magnetic field, the galaxy would collapse due to gravity,” says Brown, a professor in the University of Calgary’s School of Physics and Astronomy.
“We need to know what a galaxy’s magnetic field is like now so we can create accurate models that predict how it will evolve.”
This month, Brown and her collaborators published two studies in The Astrophysical Journal and The Astrophysical Journal Supplement Series. Their research introduces a major new dataset available to astronomers around the world, along with a new model that explains how the Milky Way’s magnetic field has changed over time.
Mapping the Milky Way magnetic field
To conduct the study, the team utilized a new telescope at the Dominion Radio Astrophysical Observatory in British Columbia, a facility run by the National Research Council of Canada. This instrument has enabled scientists to study the northern sky over a wide range of radio frequencies.
“The extensive coverage allows us to learn more about the structure of the magnetic field,” said Dr. Anna Oldog, lead author of the first study.
The observations became part of the Global Magnetic and Ionic Medium Survey (GMIMS), an international effort focused on mapping the Milky Way’s magnetic field in unprecedented detail. The result is a high-quality dataset that provides scientists with a clearer picture of the galaxy’s invisible magnetic environment.
Tracking Faraday rotation in space
The researchers collected the data by measuring an effect called Faraday rotation that occurs when radio waves pass through a region filled with electrons and magnetic fields.
“You can think of it like refraction; a straw in a glass of water will appear to bend because of how the light interacts with the material,” says Rebecca Booth, a doctoral candidate working with Brown and lead author of the second study. “Faraday rotation is a similar concept, but where electrons and magnetic fields in space interact with radio waves.”
By analyzing changes in these radio waves, the team was able to track hidden magnetic structures throughout the galaxy.
Strange magnetic reversal in Sagittarius’ arm
One of the most surprising discoveries involved the Sagittarius Arm, a region of the Milky Way where the direction of the magnetic field appears to be reversed.
“If you can look at the galaxy from above, the overall magnetic field is going clockwise,” Brown says. “But in the Sagittarius arm, it’s going counterclockwise. I couldn’t understand how that transition happened. One day Anna brought me the data and I thought, ‘Oh my god, the reversal is a diagonal!'”
Booth later expanded on Oldog’s findings using newly collected data.
“My research presents a new three-dimensional model of magnetic field reversals. From Earth, this would appear as the diagonal lines observed in the data,” Booth explains.
The discovery provides researchers with important new clues about the Milky Way’s hidden magnetic structure and could help scientists better understand how galaxies evolve over time.
