Faraday rotation signatures of fluctuation dynamos in young galaxies

TL;DR

This study uses numerical simulations to investigate how fluctuation dynamos in young galaxies produce magnetic fields that contribute to observed Faraday rotation measures, revealing the coherence and strength of these fields.

Contribution

The paper demonstrates through simulations that fluctuation dynamos can generate magnetic fields consistent with observed Faraday rotation in young galaxies, highlighting the role of turbulence regimes.

Findings

RMS RM values reach 45-55% of coherent field models at dynamo saturation.

Volume filling fields dominate RM contributions in subsonic and transonic turbulence.

Strong and overdense regions significantly influence RM in supersonic turbulence.

Abstract

Observations of Faraday rotation through high-redshift galaxies have revealed that they host coherent magnetic fields that are of comparable strengths to those observed in nearby galaxies. These fields could be generated by fluctuation dynamos. We use idealized numerical simulations of such dynamos in forced compressible turbulence up to rms Mach number of 2.4 to probe the resulting rotation measure (RM) and the degree of coherence of the magnetic field. We obtain rms values of RM at dynamo saturation of the order of 45 - 55 per cent of the value expected in a model where fields are assumed to be coherent on the forcing scale of turbulence. We show that the dominant contribution to the RM in subsonic and transonic cases comes from the general sea of volume filling fields, rather than from the rarer structures. However, in the supersonic case, strong field regions as well as moderately overdense regions contribute significantly. Our results can account for the observed RMs in young galaxies.