The formation of regular interarm magnetic fields in spiral galaxies

TL;DR

This study demonstrates that the combined effects of turbulent diffusivity contrasts and small-scale magnetic field injections can produce prominent magnetic arms in the interarm regions of spiral galaxies, aligning with observations.

Contribution

It introduces a simple no-z dynamo model showing how diffusivity gradients and magnetic injections jointly generate large-scale magnetic arms in galaxies.

Findings

Magnetic arms become more pronounced with diffusivity gradients.

Regular magnetic fields are larger in interarm regions.

The model supports the magnetic arm formation hypothesis.

Abstract

Observations of regular magnetic fields in several nearby galaxies reveal magnetic arms situated between the material arms. The nature of these magnetic arms is a topic of active debate. Previously we found a hint that taking into account the effects of injections of small-scale magnetic fields generated, e.g., by turbulent dynamo action, into the large-scale galactic dynamo can result in magnetic arm formation. We now investigate the joint roles of an arm/interarm turbulent diffusivity contrast and injections of small-scale magnetic field on the formation of large-scale magnetic field ("magnetic arms") in the interarm region. We use the relatively simple "no-$z$" model for the galactic dynamo. This involves projection on to the galactic equatorial plane of the azimuthal and radial magnetic field components; the field component orthogonal to the galactic plane is estimated from the solenoidality condition. We find that addition of diffusivity gradients to the effect of magnetic field injections makes the magnetic arms much more pronounced. In particular, the regular magnetic field component becomes larger in the interarm space compared to that within the material arms.The joint action of the turbulent diffusivity contrast and small-scale magnetic field injections (with the possible participation of other effects previously suggested) appears to be a plausible explanation for the phenomenon of magnetic arms.