Evolving Galactic Dynamos and Fits to the Reversing Rotation Measures in Edge-On Galaxies

Abstract

Rotation measure (RM) synthesis maps of NGC 4631 by Mora-Partiarroyo et al. (2018) show remarkable sign reversals on kpc scales as the distance from the minor axis increases in the northern halo of the galaxy. RM maps for edge-on galaxies observed in the CHANG-ES sample were searched through and show that regular reversals in the sign of the RM seen in galaxies appears to be a common phenomenon. These sign reversals can be naturally explained by a regular halo magnetic field that is alternating its azimuthal direction on kpc scales in the galaxy. This is a brand new phenomenon that has never before been observed in a galactic halo. Evidence of magnetic fields showing both axisymmetric and bisymmetric symmetry is found in the data.

To explain this new phenomenon the dynamo equations are solved under the assumption of scale invariance and rotating logarithmic spiral solutions are searched for. The model solutions are then compared to the observational RM map of NGC 4631 in order to draw conclusions on the type of field geometry likely found in the galactic halo. Solutions for velocity fields that represent accretion onto the disk, outflow from the disk, and rotation-only in the disk are found that produce RM maps with reversing signs viewed edge-on. Model RM maps are created for a variety of input parameters using a Faraday screen technique and are then scaled to match the amplitude of the observational maps. Residual images are then made and compared with one another in order to determine the models that provide the best fit to the data. Solutions for rotation-only, i.e. relative to a pattern uniform rotation, did in general, not fit the observational map of NGC 4631 well. Outflow models provided a reasonable fit to the magnetic field. However, the best results for the region modelled in the northern halo are found using accretion models. As there is abundant evidence for both winds and accretion in NGC 4631, this modelling technique has the potential to be able to distinguish between the dominant flows in galaxies.