Can the dynamo of spiral-arm galaxies be explained by anisotropic conductivity ?

TL;DR

This study investigates whether anisotropic electrical conductivity in spiral-arm galaxies can generate magnetic fields via dynamo effects, concluding that current galaxy structures do not satisfy the necessary conditions for such a dynamo mechanism.

Contribution

The paper analyzes the dynamo condition in spiral galaxies with anisotropic conductivity and shows that typical galaxy structures do not meet this criterion, challenging the anisotropic dynamo explanation.

Findings

Typical spiral galaxies do not satisfy the dynamo condition due to their trailing arms.

Galaxies with both trailing and leading arms also do not meet the dynamo criterion.

Numerical simulations confirm the inability of anisotropic conductivity to generate magnetic fields in observed galaxies.

Abstract

The possibility of generating a magnetic field by dynamo effect with anisotropic electrical conductivity rather than turbulent flow has been demonstrated theoretically (Plunian & Alboussi\`ere 2020) and experimentally (Alboussi\`ere et al. 2022). If the electrical conductivity is anisotropic, the electrical currents will flow preferentially in certain directions rather than others, and a simple differential rotation will suffice to generate a magnetic field. In a galaxy with spiral arms, it is reasonable to assume that the electrical conductivity will be twice larger along the arms than in the perpendicular direction, suggesting the possibility of an anisotropic dynamo. However, a further geometrical criterion must be satisfied to obtain a dynamo (Plunian & Alboussi\`ere 2022). It is given by $\Omega' \cdot\sin p > 0$, where $p$ is the pitch angle of the spiral arms, with $p \in[-\frac{\pi}{2}, \frac{\pi}{2}]$, and $\Omega'$ is the radial shear of the angular velocity. We find that the usual spiral arms galaxies, which satisfy $|\Omega'|<0$, do not satisfy this dynamo condition because they have trailing arms instead of leading arms. Even the galaxy NGC 4622, which has both trailing and leading arms, does not satisfy this dynamo condition either. This is confirmed by numerical simulations of the induction equation. Thus, for all the spiral arms galaxies known to date, the anisotropy of the spiral arms cannot explain the existence of galactic magnetic fields until further notice.