Abstract
The large-scale magnetic fields observed in spiral disc galaxies are often thought to result from dynamo action in the disc plane. However, the increasing importance of Faraday depolarization along any line of sight towards the galactic plane suggests that the strongest polarization signal may come from well above (~0.3-1 kpc) this plane, from the vicinity of the warm interstellar medium (WIM)/halo interface. We propose (see also Henriksen & Irwin 2016) that the observed spiral fields (polarization patterns) result from the action of vertical shear on an initially poloidal field. We show that this simple model accounts for the main observed properties of large-scale fields. We speculate as to how current models of optical spiral structure may generate the observed arm/interarm spiral polarization patterns.
Highlights
Large–scale magnetic fields are found in all disc galaxies by observing radio synchrotron emission
This is caused by the pinching of an initial global vertical field by the radial inward/outward motion of the warm interstellar medium (WIM) gas/halo gas, respectively, (ii) Galaxies seen face-on show spiral magnetic patterns
These are caused by the twisting of the initial global vertical field by the rotation of the galactic disk (WIM), and (iii) In galaxies seen at intermediate angles, and at longer wavelengths (λ ∼ 20 cm) we observe a reduction in the polarized emission on the receding end of the major axis
Summary
Large–scale magnetic fields are found in all disc galaxies by observing radio synchrotron emission. Analysis of the polarization properties of the radio emission shows that ordered fields with spiral patterns are found in grand–design, barred and flocculent galaxies. (i) Spiral field patterns are seen in almost all galaxies at wavelengths λ ∼ 3 − 6 cm, where Faraday depolarization by the halo is negligible. They are seen even in galaxies lacking (strong) optical spiral structure – for example NGC 4736 (Chyzy & Buta 2008; Beck & Wielebinski 2013). Further Terral & Ferriere (2017) provide evidence that the magnetic field in the Galactic halo has the required
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