Galactic spirals and dynamo action: Slaved non-axisymmetric modes

Luke Chamandy (Inter-University Centre for Astronomy and Astrophysics) India, lchamandy@gmail.com\\ Kandaswamy Subramanian (Inter-University Centre for Astronomy and Astrophysics) kandu@iucaa.ernet.in\\ Anvar Shukurov (University of Newcastle) anvar.shukurov@newcastle.ac.uk

Abstract
We present a model to account for the phenomenon of coherent large-scale magnetic arms observed in many nearby disc galaxies (and possibly in our own), by invoking non-axisymmetric forcing of a mean field dynamo. Unlike previous work, our treatment is generalized to include non-local effects in time using the minimal $\tau$ approximation (MTA). For the first time in such non-axisymmetric disc dynamo models, we incorporate the dynamical quenching formalism. Moreover, we examine the structure of the magnetic fields produced by matter spirals that are not only steady but also transient. For steady forcing, we find magnetic spiral patterns that are slaved to the axisymmetric modes, strongly peaked around corotation, and go from leading the matter spiral inside corotation to trailing it outside. We show that the $\tau$-effect can phase shift the magnetic spiral to make it mostly trailing. In the non-linear regime the magnetic field is able to spread radially to encompass the disc, provided that there exists a vertical magnetic helicity flux due to say a wind. The absence of such a wind leads to expanding and contracting rings of magnetic field in the disc. Starting with large random seeds can sometimes lead to global reversals. The onset of transient matter spirals rapidly leads to magnetic spirals. The $\tau$-effect significantly increase the time for the magnetic spiral to decay even after the sudden destruction of the matter spiral, leading to ghost magnetic spirals. It can also lead to a large phase shift between the magnetic and matter arms, reminiscent of what is seen in the galaxy NGC6946.