Synthesizing Observations and Theory to Understand Galactic Magnetic Fields: Progress and Challenges

TL;DR

This paper reviews observational data and dynamo theory related to galactic magnetic fields, highlighting progress, challenges, and future prospects with upcoming large-scale surveys like SKA.

Contribution

It compiles and analyzes galactic magnetic field properties, clarifies differences between observations and models, and evaluates dynamo theory predictions against new data.

Findings

No evidence for spiral modulation of turbulent speed in studied galaxies.

Summarizes current knowledge on magnetic field properties and dynamo models.

Identifies challenges and proposes strategies for future research with SKA.

Abstract

Constraining dynamo theories of magnetic field origin by observation is indispensable but challenging, in part because the basic quantities measured by observers and predicted by modelers are different. We clarify these differences and sketch out ways to bridge the divide. Based on archival and previously unpublished data, we then compile various important properties of galactic magnetic fields for nearby spiral galaxies. We consistently compute strengths of total, ordered, and regular fields, pitch angles of ordered and regular fields, and we summarize the present knowledge on azimuthal modes, field parities, and the properties of non-axisymmetric spiral features called magnetic arms. We review related aspects of dynamo theory, with a focus on mean-field models and their predictions for large-scale magnetic fields in galactic discs and halos. Further, we measure the velocity dispersion of HI gas in arm and inter-arm regions in three galaxies, M 51, M 74, and NGC 6496, since spiral modulation of the root-mean-square turbulent speed has been proposed as a driver of non-axisymmetry in large-scale dynamos. We find no evidence for such a modulation and place upper limits on its strength, helping to narrow down the list of mechanisms to explain magnetic arms. Successes and remaining challenges of dynamo models with respect to explaining observations are briefly summarized, and possible strategies are suggested. With new instruments like the Square Kilometre Array (SKA), large data sets of magnetic and non-magnetic properties from thousands of galaxies will become available, to be compared with theory.