Dynamo saturation in direct simulations of the multi-phase turbulent interstellar medium

TL;DR

This study investigates how initial magnetic field configurations influence the saturation of the mean-field dynamo in the turbulent interstellar medium through direct numerical simulations, revealing agreement with algebraic quenching models and implications for galaxy magnetic fields.

Contribution

It demonstrates that the saturated magnetic field in DNS aligns with a quenched mean-field dynamo model, highlighting the role of supernova rates and galactic winds in magnetic field saturation.

Findings

Final magnetic field strength matches quenched $ ext{αΩ}$ dynamo predictions.

Supernova rates influence magnetic field saturation via galactic winds.

Mean field algebraic quenching can serve as a sub-grid model in galaxy simulations.

Abstract

The ordered magnetic field observed via polarized synchrotron emission in nearby disc galaxies can be explained by a mean-field dynamo operating in the diffuse interstellar medium (ISM). Additionally, vertical-flux initial conditions are potentially able to influence this dynamo via the occurrence of the magneto-rotational instability (MRI). We aim to study the influence of various initial field configurations on the saturated state of the mean-field dynamo. This is motivated by the observation that different saturation behavior was previously obtained for different supernova rates. We perform direct numerical simulations (DNS) of three-dimensional local boxes of the vertically stratified, turbulent interstellar medium, employing shearing-periodic boundary conditions horizontally. Unlike in our previous work, we also impose a vertical seed magnetic field. We run the simulations until the growth of the magnetic energy becomes negligible. We furthermore perform simulations of equivalent 1D dynamo models, with an algebraic quenching mechanism for the dynamo coefficients. We compare the saturation of the magnetic field in the DNS with the algebraic quenching of a mean-field dynamo. The final magnetic field strength found in the direct simulation is in excellent agreement with a quenched $\alpha\Omega$~dynamo. For supernova rates representative of the Milky Way, field losses via a Galactic wind are likely responsible for saturation. We conclude that the relative strength of the turbulent and regular magnetic fields in spiral galaxies may depend on the galaxy's star formation rate. We propose that a mean field approach with algebraic quenching may serve as a simple sub-grid scale model for galaxy evolution simulations including a prescribed feedback from magnetic fields.