Computational approaches to modeling dynamos in galaxies

TL;DR

This paper reviews computational methods for modeling galactic magnetic fields, highlighting how small-scale turbulent dynamos and large-scale dynamos contribute to observed magnetic structures and strengths.

Contribution

It provides a comprehensive overview of numerical modeling techniques and recent findings on dynamo processes in galaxy formation and evolution.

Findings

Small-scale turbulent dynamos operate during galaxy formation, amplifying magnetic fields rapidly.

Large-scale dynamos produce observed galactic magnetic field structures and strengths.

Models align with observations like Faraday rotation and synchrotron polarization.

Abstract

Galaxies are observed to host magnetic fields with a typical total strength of around 15microgauss. A coherent large-scale field constitutes up to a few microgauss of the total, while the rest is built from strong magnetic fluctuations over a wide range of spatial scales. This represents sufficient magnetic energy for it to be dynamically significant. Several questions immediately arise: What is the physical mechanism that gives rise to such magnetic fields? How do these magnetic fields affect the formation and evolution of galaxies? In which physical processes do magnetic fields play a role, and how can that role be characterized? Numerical modelling of magnetized flows in galaxies is playing an ever-increasing role in finding those answers. We review major techniques used for these models. Current results strongly support the conclusion that field growth occurs during the formation of the first galaxies on timescales shorter than their accretion timescales due to small-scale turbulent dynamos. The saturated small-scale dynamo maintains field strengths at a few percent of equipartition with turbulence. The subsequent action of large-scale dynamos in differentially rotating discs produces observed modern field strengths in equipartition with the turbulence and having power at large scales. The field structure resulting appears consistent with observations including Faraday rotation and polarisation from synchrotron and dust thermal emission. Major remaining challenges include scaling numerical models toward realistic scale separations and Prandtl and Reynolds numbers.