Inner view of the Milky Way’s magnetic field shows spiral structure

March 01, 2022

Magnetic fields of spiral galaxies usually show spiral structures themselves. For our own galaxy, this could not be confirmed so far due to our unfavorable inner perspective. Researchers at the Max Planck Institute for Astrophysics have now shown that the local galactic magnetic field is indeed aligned with the local Orion spiral arm of the Milky Way, as expected for a spiral galaxy.

Magnetic fields pervade space almost unnoticed. This is not only true for the terrestrial magnetic field, which we humans cannot perceive directly with any of our senses, but also for the magnetic field of the Milky Way. Starlight and other radiation pass practically unchanged through the cosmic magnetic fields, but they leave subtle traces in linearly polarized radio waves. The plane of the waves’ polarization rotates thanks to the Faraday effect produced by free electrons in combination with magnetic fields in interstellar space. The rotation gets greater for stronger magnetic fields in the direction of propagation of the wave, if more electrons are present, and for longer wavelengths of the radiation. This latter relationship allows us to determine the mean galactic magnetic field along the line-of-sight based on observations of extragalactic radio sources at various wavelengths.

An international team led by Sebastian Hutschenreuter and Torsten Enßlin at the Max Planck Institute for Astrophysics (MPA) converted such measurements of single radio sources to a sky map of the galactic Faraday effect (Fig. 1). A special algorithm extracts the galactic part from individual measurements of the Faraday effect and calculates a continuous image of the whole sky.

The resulting map showed that the strength of the observed Faraday effect follows largely the known structures of the so-called bremsstrahlung of free electrons (Fig. 2). This was to be expected, since the same free electrons produce the Faraday effect; but the sky distribution of the strength of the Faraday effect could not entirely be explained by free electrons. The data contained another signal characterized by two roundish regions located at opposite directions on the sky (Fig. 2, bottom).

These positions of the enhanced Faraday effect correspond to those of the Orion spiral arm, which includes our solar system (Fig. 3). The increased signal in these directions cannot be due to additional electrons, since they should show up as bremsstrahlung as well. A plausible explanation would be an alignment of the local magnetic fields with the direction of the Orion spiral arm. This would also fit well with the magnetic fields of other galaxies that are aligned with the spiral arms. Additional support for this interpretation comes from the lack of polarization of intergalactic dust in these directions, which indicates a weak magnetic field component perpendicular to the spiral arm.

The discovery of the magnetic spiral structure, at least in the direct vicinity of the Sun, was only possible by merging a wide variety of data sets with information field theory developed at MPA in the research group of Torsten Enßlin. IFT allows to incorporate certain physical correlations, such as the one between the Faraday effect and bremsstrahlung which both are based on free electrons, into the data analysis, while remaining open to surprises in the data.

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