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

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 Based on archival and previously unpublished data, we then compile various important properties of galactic magnetic fields for nearby spiral galaxies.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 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.

Chang-Es

“…The halo magnetic field is roughly constant at a value of 7.4 µG. These values are in good agreement with the values of other spiral galaxies (Beck et al 2019).…”

“…Polarization from radio continuum observations of star-forming galaxies often show X-shaped magnetic field structures if observed edgeon (Tüllmann et al 2000;Krause 2009). One proposed mechanism to maintain these large-scale fields is the mean-field α − ω dynamo (see e.g., Ruzmaikin et al 1988;Beck et al 1996Beck et al , 2019Chamandy 2016;Henriksen 2017;Henriksen et al 2018), where turbulent magnetic fields in spiral galaxies are amplified and ordered by the α-process (driven by e.g., supernova explosions and the Coriolis force), combined with the ω-process (shear motions due to differential rotation) to result in large-scale regular magnetic fields (e.g., Arshakian et al 2009).…”

The magnetized disk-halo transition region of M 51

“…This would provide knowledge on the field properties in the disk and halo of M 51 and other nearby spiral galaxies with almost face-on orientation. We also need an improved dynamo model for M 51 that includes tidal forces and a significant halo component (see Chapter 8.2 in Beck et al 2019).…”

Chang-Es

“…The magnetic fields in the disks of spiral galaxies have been studied extensively (Wielebinski et al 2010;Krause 2019;Beck et al 2019). They consist of a turbulent magnetic field component of scales ranging from a few to a few hundred parsecs and a large-scale magnetic field that is found to form a spiral pattern parallel to the midplane of the galaxy.…”