Giant magnetic toroids found in Milky Way’s Halo

The distribution of Faraday rotation measurements (RMs) of radio sources outside the Galaxy and radio pulsars inside the Galaxy can provide information on the magnetic fields in our Milky Way. The magnetic field toroids above and below the Galactic plane with reversed field directions exist in the Galactic halo and have been included in almost all models for the global magnetic structure in the Milky Way.

This is because they are based on the antisymmetry of the Faraday sky in the inner Galaxy to the Galactic coordinates. Nevertheless, it is challenging to calculate the quantitative characteristics from observational data, such as the scale height, scale radius, and field strength of the toroids.

In a new study, Dr. XU Jun and Prof. HAN Jinlin of the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) discovered huge magnetic toroids in the Milky Way’s halo. These toroids are essential for propagating cosmic rays and provide vital constraints on the physical processes in the interstellar medium and the genesis of cosmic magnetic fields.

Prof. HAN, a leading expert in this field of study, has measured the polarization of pulsars and their Faraday effects over an extended period of time to estimate the magnetic field structures along the spiral arms of the Galactic disk.

In 1997, he discovered a remarkable antisymmetry in the Faraday effects of cosmic radio sources in the sky regarding the coordinates of the Milky Way galaxy. This indicates that the magnetic fields in the Milky Way’s halo have a toroidal field structure, with the directions of the magnetic fields reversed below and above the Galactic plane.

But for decades, astronomers have struggled to measure the size of these toroids or the intensity of their magnetic fields. Since pulsars and a few other nearby radio-emission objects exhibit Faraday effects consistent with antisymmetry, they reasoned that the antisymmetry of the sky distribution of Faraday effects of radio sources could be caused only by the interstellar medium in the vicinity of the Sun.

The crux demonstrates whether or not toroidal structures in magnetic fields existed beyond the Sun’s immediate vicinity in the massive Galactic halo.

In this study, Prof. HAN suggested that the contribution from measurements of background cosmic sources could be subtracted from the counting of the Faraday rotation from the interstellar medium near the Sun. A significant number of pulsars have recently been measured, some of which were obtained by the Five-hundred Aperture Spherical Radio Telescope (FAST) on its own. 

Dr. XU gathered the Faraday rotation measurement data during the last 30 years. Scientists discovered through data analysis that the antisymmetry of the Faraday rotation measurements caused by the Galactic halo exists throughout the sky, from the Milky Way’s center to its anti-center. This suggests that the toroidal magnetic fields with such an odd symmetry are vast, ranging in radius from 6000 to 50,000 light-years from the Milky Way’s center.

This study provides humans with a new understanding of the physics of our Milky Way and is a landmark for research on cosmic magnetic fields.

Journal Reference:

J. Xu and J. L. Han. The Huge Magnetic Toroids in the Milky Way Halo. The Astrophysical Journal. DOI 10.3847/1538-4357/ad3a61

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