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 Based on archival and previously unpublished data, we then compile various important properties of galactic magnetic fields for nearby spiral galaxies.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. Furthermore, we measure the velocity dispersion of H i gas in arm and inter-arm regions in three galaxies, M 51, M 74, and NGC 6946, 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.
Highlights
The presence of magnetic fields in the interstellar medium (ISM) of the Milky Way and external galaxies has been known for more than 60 years
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
Faraday depolarization strongly decreases with decreasing wavelength, so that the polarized emission should become smooth and the magnetic arms should disappear at 3.6 cm, which is not observed
Summary
The presence of magnetic fields in the interstellar medium (ISM) of the Milky Way and external galaxies has been known for more than 60 years. Answering these questions covers very broad ground and we touch on aspects of all of them, we focus here on the curious fact that the magnetic field of spiral galaxies often exhibits a large-scale component with net flux of one sign over a large portion of the galactic area, even when there is a smaller scale random component. (4) What might we learn from their properties about galaxy structure or dynamical processes? Galaxies 2020, 8, 4 diffusion represents exponential decay, the growth and replenishment must itself supply exponential growth This fact has led to a grand enterprise of in-situ galactic dynamo theory and modeling to explain the large-scale fields. The purpose of our review is to bring the reader up to date on the efficacy with which large-scale dynamo theory and observation are consistent and where challenges remain
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have