The magnetic fields in the Milky Way’s halo have a toroidal structure extending from 6,000 light-years to 50,000 light-years from the Galactic center. The Sun is about 30,000 light years away. Credit: NAOC
Astrophysicists have discovered large magnetic toroids in Milky Waya halo that affects the propagation of cosmic rays and the physics of interstellar space. Their research, based on extensive Faraday spin data, reveals that these toroids extend across the galaxy, confirming the presence of significant toroidal magnetic fields.
The origin and evolution of cosmic magnetic fields is a long-standing open question at the frontier of astronomical and astrophysical research and has been chosen as one of the key research areas for many large world-class radio telescopes, including the Square Kilometer Array (SKA) under construction. Determining the large-scale structures of the magnetic field in the Milky Way has been a major challenge for many astronomers around the world for decades.
Detection of magnetic toroids
In a new study published in The Astrophysical Journal on May 10, Dr. Jun Xu and Prof. Jinlin Han of the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) discovered huge magnetic toroids in the Milky Way’s halo, which are fundamental to the propagation of cosmic rays and provide a crucial constraint on physical processes in the interstellar medium and the origin of cosmic magnetic fields.
Prof. Han, a leading scientist in this research area, has determined the magnetic field structures along the spiral arms of the galactic disk through a long-term project to measure the polarization of pulsars and their Faraday effects. In 1997 he discovered a striking antisymmetry of the Faraday effects of cosmic radio sources in the sky with respect to the coordinates of our Milky Way galaxy, indicating that the magnetic fields in the Milky Way halo have a toroidal field structure, with reversed magnetic field directions below and above the galactic plane.
Challenges in measuring magnetic fields
However, determining the size of these toroids or the strength of their magnetic fields has been a difficult task for astronomers for decades. They suspected that the antisymmetry of the sky distribution of the Faraday effects of radio sources could only be produced by the interstellar medium near the Sun, since pulsars and some nearby radio-emitting objects that are quite close to the Sun show Faraday Effects consistent with the antisymmetry. The key is to show whether the magnetic fields in the vast galactic halo had such a toroidal structure outside the vicinity of the Sun.
Innovative research methods
In this study, Prof. Hahn innovatively proposed that the Faraday spin of the interstellar medium near the Sun could be counted through the measurements of a large number of pulsars, some of which were obtained recently by the Five Hundred Aperture Spherical Radio Telescope (FAST) alone, and then could the contribution from measurements of background cosmic sources was subtracted. All Faraday rotation measurement data over the past 30 years has been collected by Dr. Xu.
Through data analysis, the scientists found that the antisymmetry of Faraday’s spin measurements, caused by the environment in the galactic halo, exists across the entire sky, from the center to the anticenter of our Milky Way, suggesting that toroidal magnetic fields with such strange symmetry have huge dimensions existing within a radius of 6,000 light years to 50,000 light years from the center of the Milky Way.
Conclusion and impact
This research greatly improved our understanding of the physics of the Milky Way and marked a milestone in the study of cosmic magnetic fields.
Reference: “The giant magnetic toroids in the Milky Way’s halo” by J. Xu and JL Han, 10 May 2024, The Astrophysical Journal.
DOI: 10.3847/1538-4357/ad3a61