Our galaxy has a weak – but immense – magnetic field that extends across most of it. Although we know the broad outlines of it, the fine details are a mystery. Now the magnetism, or a small part of it, has been revealed at finer resolution, revealing that it is much more jumbled up than previously suggested smooth models.
The galactic magnetic field isn’t strong enough to stick anything to your refrigerator, let alone generate electricity from a turbine. Nevertheless, it determines the way stars and planets form by causing the raw material to clump more than gravity itself would. The field polarizes the light passing through it, which is how we detected and measured it.
Unfortunately, when we look across the galaxy, we see a combined effect of all the fields in our line of sight, rather than a three-dimensional map.
“Until now, all observations of magnetic fields in the Milky Way led to a very limited model that was uniform everywhere and largely matched the disk shape of the galaxy itself,” study author Dr. Yasuo Doi of the University of Tokyo said in a statement. .
The fact that stars and planets can produce local fields that are much stronger (and usually in different directions) than the galactic field is known, and the field around some stars has been measured. However, there was a gap between detecting specific local fields and the large-scale shape, with little idea of what the field looked like on a scale of tens or hundreds of light years.
Doi and colleagues combined data from the Gaia satellite and measurements of polarized light on Earth to find signs of magnetism on smaller scales. It would be an epic task to do this across the entire galaxy. So the team focused on part of the Sagittarius Arm, one of the galaxy’s four major spiral arms. The Sun and Earth are in the smaller Orion-Cygnus arm, possibly an offshoot of the large Perseus arm, but it’s much harder to map something you’re in compared to mapping a neighbor .
The team measured the polarization of hundreds of stars within their chosen field and used Gaia to accurately locate these stars. This allowed them to identify the contributions of five huge clouds of magnetized gas in the field.
Each cloud has a field that is smooth on a scale of 15-30 light-years and larger, but often oriented very differently from the galaxy as a whole.
The white lines show the polarization, which correlates with the orientation of local magnetic field lines. It reveals that the galactic field is far from homogeneous. Combined, this information forms a detailed map of the magnetic field in the Sagittarius arm of the Milky Way.
Three clouds within the Sagittarius arm have fields with broadly similar alignments to each other (40°-58° away from galactic north), but another cloud is roughly perpendicular to these three. A fifth cloud, lying between us and the Sagittarius arm, has an angle similar to the outlier beneath the Sagittarius clouds. That brings the clouds out of alignment with the galactic plane by up to 60°, with which the galactic magnetic field is presumably aligned. Their direction likely reflects the effects of a major past event, such as an ancient supernova explosion that left a magnetic legacy.
“Personally, I am intrigued by the fundamental process of star formation, crucial for the origin of life, including ourselves, and I strive to understand this phenomenon in its entirety over time,” Doi said. To do this, he believes it is necessary to better understand galactic magnetic field lines, and he hopes to be able to map more of the way gas accumulates prior to the birth of stars.
The research has been published open access in The Astrophysical Journal.