Can galactic magnetic fields diffuse into the voids?

TL;DR

This paper investigates whether galactic magnetic fields can diffuse into cosmic voids and concludes that diffusion is too slow, suggesting primordial origins are more likely for void magnetization.

Contribution

It demonstrates that plasma diffusion limits galactic magnetic field spread into voids, favoring primordial origins for observed void magnetization.

Findings

Diffusion of galactic fields into voids is too slow due to plasma conductivity.

Galactic fields only diffuse out to 200-400 kpc in voids.

Primordial magnetic fields are the most plausible explanation for void magnetization.

Abstract

Cosmic voids are magnetized at the level of at least $10^{-17}$ G on Mpc scales, as implied by blazar observations. We show that an electrically conducting plasma is present in the voids, and that, because of the plasma, \emph{diffusion} into the voids of galactic fields generated by a mean-field dynamo is far too slow to explain the present-day void magnetization. Indeed, we show that even in the presence of turbulence in the voids, dynamo-generated galactic fields diffuse out to a galactocentric radius of only 200-400 kpc. Therefore, it is challenging to meet the required volume filling-factor of the void magnetic field. We conclude that a primordial origin remains the most natural explanation to the space-filling weak fields in voids.