A physical approach to modelling large-scale galactic magnetic fields

Abstract

Context.A convenient representation of the structure of the large-scale galactic magnetic field is required for the interpretation of polarization data in the sub-mm and radio ranges, in both the Milky Way and external galaxies. Aims. We develop a simple and flexible approach to construct parametrised models of the large-scale magnetic field of the Milky Way and other disc galaxies, based on physically justifiable models of magnetic field structure. The resulting models are designed to be optimised against available observational data. Methods. Representations for the large-scale magnetic fields in the flared disc and spherical halo of a disc galaxy were obtained in the form of series expansions whose coefficients can be calculated from observable or theoretically known galactic properties. The functional basis for the expansions is derived as eigenfunctions of the mean-field dynamo equation or of the vectorial magnetic diffusion equation.Results. The solutions presented are axially symmetric but the approach can be extended straightforwardly to non-axisymmetric cases. The magnetic fields are solenoidal by construction, can be helical, and are parametrised in terms of observable properties of the host object, such as the rotation curve and the shape of the gaseous disc. The magnetic field in the disc can have a prescribed number of field reversals at any specified radii. Both the disc and halo magnetic fields can separately have either dipolar or quadrupolar symmetry. The model is implemented as a publicly available software package galmag which allows, in particular, the computation of the synchrotron emission and Faraday rotation produced by the model's magnetic field. Conclusions. The model can be used in interpretations of observations of magnetic fields in the Milky Way and other spiral galaxies, in particular as a prior in Bayesian analyses. It can also be used for a simple simulation of a time-dependent magnetic field generated by dynamo action.

Helicity in the large-scale Galactic magnetic field

“…Improved Galactic magnetic field modelling is also necessary to verify these results. The IMAGINE consortium and the Bayseian inference code they are developing (Haverkorn et al 2019) aims to develop more sophisticated Galactic magnetic field modelling using dynamo models such as those in GALMAG (Shukurov et al 2019), which include higher-order dynamo modes and combinations of modes. The cross-correlation analysis presented in this work should be applied to an improved model for the Galactic magnetic field when such a model becomes available.…”

A dynamo amplifying the magnetic field of a Milky-Way-like galaxy

“…For example, Stix (1975) showed that in a disk geometry a quadrupolar field should grow faster, while a dipolar field can arise as a result of asymmetries. Shukurov et al (2019) also presented a series of models where quadrupolar fields are favored in the disk, while dipolar geometries arise in the halo. In order to compare our findings to these analytical estimates, we need to split the magnetic field into a mean and a turbulent component.…”

On the origin of magnetic driven winds and the structure of the galactic dynamo in isolated galaxies

“…Elmegreen & Scalo 2004). Further, theoretical calculations can predict the structure of the magnetic field which turns out to be either dipolar or quadrupolar (Shukurov et al 2019), whereby the quadrupolar structures decay faster if the dynamo action is switched off. The field structure can then be determined by the symmetry of the magnetic field around the mid plane, where uneven symmetry determines a dipolar field while even symmetry indicates a quadrupolar field.…”