The relation between magnetic and material arms in models for spiral galaxies

Abstract

Context.Observations of polarized radio emission show that large-scale (regular) magnetic fields in spiral galaxies are not fully axisymmetric, but generally stronger in interarm regions. In some nearby galaxies such as NGC 6946 they are organized in narrow magnetic arms situated between the material spiral arms. Aims. The phenomenon of magnetic arms and their relation to the optical spiral arms (the material arms) calls for an explanation in the framework of galactic dynamo theory. Several possibilities have been suggested but are not completely satisfactory; here we attempt a consistent investigation. Methods. We use a 2D mean-field dynamo model in the no-z approximation and add injections of small-scale magnetic field, taken to result from supernova explosions, to represent the effects of dynamo action on smaller scales. This injection of small scale field is situated along the spiral arms, where star-formation mostly occurs. Results. A straightforward explanation of magnetic arms as a result of modulation of the dynamo mechanism by material arms struggles to produce pronounced magnetic arms, at least with realistic parameters, without introducing new effects such as a time lag between Coriolis force and α-effect. In contrast, by taking into account explicitly the small-scale magnetic field that is injected into the arms by the action of the star forming regions that are concentrated there, we can obtain dynamo models with magnetic structures of various forms that can be compared with magnetic arms. These are rather variable entities and their shape changes significantly on timescales of a few 100 Myr. Properties of magnetic arms can be controlled by changing the model parameters. In particular, a lower injection rate of small-scale field makes the magnetic configuration smoother and eliminates distinct magnetic arms. Conclusions. We conclude that magnetic arms can be considered as coherent magnetic structures generated by large-scale dynamo action, and associated with spatially modulated small-scale magnetic fluctuations, caused by enhanced star formation rates within the material arms.

Resolved magnetic structures in the disk-halo interface of NGC 628

“…The dynamo models by Moss et al (2013Moss et al ( , 2015 assume that a large-scale ordered field is generated everywhere in the disk, while a small-scale dynamo injects turbulent fields only in the spiral arms. This gives polarisation arms between the gaseous arms at all radii but with pitch angles of the polarisation structures and pitch angles of the magnetic field lines that are significantly smaller than those of the gaseous arms.…”

Magnetic and gaseous spiral arms in M83

“…The mean-field dynamo action can be suppressed within the gaseous arms by either a presumably enhanced fluctuation dynamo driven by stronger star formation (Moss et al 2013) or a stronger galactic outflow driven by stronger star formation (Sur et al 2007;Chamandy et al 2015; see also Shukurov 1998). A generic problem of such mechanisms is that they produce the desired displacement only within a few kiloparsecs of the corotation radius of the spiral pattern (Shukurov 1998;Chamandy et al 2013Chamandy et al , 2014, because the residence time of a volume element within a gaseous spiral arm is shorter than the dynamo time scale of order 5 × 10 8 yr at large distances from the corotation.…”

Magnetic fields in primordial accretion disks

“…They even become dynamically important, launch outflows, and constrain the star formation efficiency. Various studies (Ruzmaikin et al 1988;Schmitt 1990;Kulsrud 1999;Moss et al 2012Moss et al , 2013 suggest that the large-scale dynamo is essential to generate ordered magnetic fields observed in spiral galaxies. We therefore emphasize that such dynamo processes have not only been confirmed in idealized scenarios, but in simulations of real astrophysical systems.…”