On the contribution of galaxies to the magnetic field in cosmic voids

TL;DR

The paper assesses whether astrophysical processes can generate magnetic fields in cosmic voids by examining the screening effect of intergalactic plasmas on dipole-like fields from galaxies. It derives a Fourier-space evolution for $\mathbf{B}_{\mathbf{k}}$ under resistive, non-flow conditions and defines a magnetic screening length $l_{r,0}$ from $k_r(0)$, finding $\mathbf{B}_{\mathbf{k}}(0)=4\pi[1-\exp(-k^2/k_{r,0}^2)]\mathbf{M}_{\mathbf{k}}^{\perp}(0)$. With plausible parameters (e.g., Spitzer resistivity and $T_{e,0}\sim 10$ eV), the screening length is tiny ($l_{r,0}\sim 2\times 10^{-4}$ pc), causing exponential suppression of large-scale fields $r\gg l_{r,0}$ and making void-field strengths unable to reach the observational lower bounds, unless $l_{r,0}$ is unrealistically large ($\sim$ few Mpc). Consequently, dipole superposition from galaxies cannot account for the inferred $B_{ m void}$ limits, and other mechanisms—such as magnetized outflows or plasma instabilities—likely contribute. The work provides a qualitative demonstration of screening and motivates more detailed treatments of plasma effects in the intergalactic medium.

Abstract

Astrophysical processes can contribute to magnetic fields within cosmic voids either through magnetized outflows from the astrophysical large-scale structure or through superposition of dipolar contributions from individual galaxies. Such astrophysical magnetic fields represent a foreground to possible space-filling primordial magnetic fields seeded in the early Universe. In this paper, we provide a qualitative description of the screening of magnetic fields by intergalactic plasmas. We find that contributions from superposition of static dipoles are highly suppressed and cannot explain indications for lower bounds based on observations of $γ$-ray cascades from high energy sources such as blazars.