For hundreds of years, astronomers have studied the night sky in an effort to understand the forces shaping 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 are producing one of the clearest views yet of that hidden structure.
“Without a magnetic field, the galaxy would collapse in on itself due to gravity,” says Brown, a professor in the Department of Physics and Astronomy at the University of Calgary.
“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.”
This month, Brown and her collaborators published two studies in The Astrophysical Journal and The Astrophysical Journal Supplement Series. Their work introduces a major new dataset that astronomers around the world will be able to use, along with a new model explaining how the Milky Way’s magnetic field may have changed over time.
Mapping the Milky Way’s Magnetic Field
To carry out the research, the team relied on a new telescope at the Dominion Radio Astrophysical Observatory in British Columbia, a facility operated by the National Research Council Canada. The instrument allowed scientists to survey the northern sky across a wide range of radio frequencies.
“The broad coverage really lets you get at the details about the magnetic field structure,” says Dr. Anna Ordog, PhD, lead author of the first study.
The observations became part of the Global Magneto-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 much 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, which occurs when radio waves travel through areas filled with electrons and magnetic fields.
“You can think of it like refraction. A straw in a glass of water looks bent because of how light interacts with matter,” says Rebecca Booth, a PhD candidate working with Brown and lead author of the second study. “Faraday rotation is a similar concept, but it’s electrons and magnetic fields in space interacting with radio waves.”
By analyzing these changes in radio waves, the team was able to trace hidden magnetic structures throughout the galaxy.
A Strange Magnetic Reversal in the Sagittarius Arm
One of the most surprising discoveries involved the Sagittarius Arm, a region of the Milky Way where the magnetic field appears to reverse direction.
“If you could look at the galaxy from above, the overall magnetic field is going clockwise,” says Brown. “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 later expanded on Ordog’s finding using the newly collected data.
“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,” Booth explains.
The discovery provides researchers with an important new clue about the hidden magnetic architecture of the Milky Way and could help scientists better understand how galaxies evolve over time.