Nonlinear Galactic Dynamos and the Magnetic Pitch Angle

TL;DR

This paper investigates the discrepancy between observed large-scale magnetic pitch angles in spiral galaxies and predictions from standard mean-field dynamo theory, exploring parameter space and suggesting extensions to the model.

Contribution

The study identifies key dynamo parameters influencing pitch angles and demonstrates that standard models cannot fully account for observations, proposing directions for model improvements.

Findings

Predicted pitch angles can reach ~40°, matching some observations.

Most parameter space predicts smaller angles, indicating a need for additional physics.

Non-standard parameter values are plausible but lie near the boundary of the allowed space.

Abstract

Pitch angles $p$ of the large-scale magnetic fields $\overline{\bf{\it{B}}}$ of spiral galaxies have previously been inferred from observations to be systematically larger in magnitude than predicted by standard mean-field dynamo theory. This discrepancy is more pronounced if dynamo growth has saturated, which is reasonable to assume given that such fields are generally inferred to be close to energy equipartition with the interstellar turbulence. This 'pitch angle problem' is explored using local numerical mean-field dynamo solutions as well as asymptotic analytical solutions. It is first shown that solutions in the saturated or kinematic regimes depend on only five dynamo parameters, two of which are tightly constrained by observations of galaxy rotation curves. The remaining 3-dimensional (dimensionless) parameter space can be constrained to some extent using theoretical arguments. Predicted values of $|p|$ can be as large as $\sim40^\circ$, which is similar to the largest values inferred from observations, but only for a small and non-standard region of parameter space. We argue, based on independent evidence, that such non-standard parameter values are plausible. However, these values are located toward the boundary of the allowed parameter space, suggesting that additional physical effects may need to be incorporated. We therefore suggest possible directions for extending the basic model considered.