The small scale dynamo and the amplification of magnetic fields in massive primordial haloes

TL;DR

This study uses high-resolution cosmological simulations to demonstrate that the small-scale dynamo can rapidly amplify magnetic fields in primordial galaxy formation, reaching significant strengths early on.

Contribution

It provides the first detailed simulation evidence that the small-scale dynamo operates efficiently in primordial halos with high Jeans resolution, leading to early magnetic field saturation.

Findings

Dynamo action confirmed at resolutions ≥64 cells per Jeans length.

Magnetic fields reach approximately 0.1 μG at densities of 10^4 cm^{-3}.

Saturation occurs early due to high Reynolds numbers in primordial galaxies.

Abstract

While present standard model of cosmology yields no clear prediction for the initial magnetic field strength, efficient dynamo action may compensate for initially weak seed fields via rapid amplification. In particular, the small-scale dynamo is expected to exponentially amplify any weak magnetic field in the presence of turbulence. We explore whether this scenario is viable using cosmological magneto-hydrodynamics simulations modeling the formation of the first galaxies, which are expected to form in so-called atomic cooling halos with virial temperatures $\rm T_{vir} \geq 10^{4}$ K. As previous calculations have shown that a high Jeans resolution is needed to resolve turbulent structures and dynamo effects, our calculations employ resolutions of up to 128 cells per Jeans length. The presence of the dynamo can be clearly confirmed for resolutions of at least 64 cells per Jeans length, while saturation occurs at approximate equipartition with turbulent energy. As a result of the large Reynolds numbers in primordial galaxies, we expect saturation to occur at early stages, implying magnetic field strengths of \sim0.1 $\mu$G at densities of 10^4 cm^{-3}.