Galactic dynamos and density wave theory*

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Abstract
A steady density wave in the stellar background of a disklike galaxy is supposed to force a spiral shock wave in the interstellar gas, yielding a corresponding spiral form for the crucial ‘α-effect’ term in standard dynamo theory. The thin-disc approximation to the dynamo equations is extrapolated to apply to cases with strong departures from axisymmetry. The equations in truncated form are studied both by WKBJ methods and by numerical solution. Rapidly growing, global, bisymmetric ( m = ± 1) magnetic modes are found, corotating with the density wave. For the chosen parameters they extend for 3–4 kpc around the radius rc where the wave corotates with the gas. This may be compared with the domain, located nearer to the Galactic Centre and of just 1-kpc extent, found when the α-effect is taken as axisymmetric. The magnetic spiral is found to be closely correlated with the density wave, with its dominant part leading within rc and trailing outside. Similar studies of even ( m = 0, ± 2) modes yield several which are essentially axisymmetric with small m = ± 2 perturbations, and some with comparable m = 0 and m = ± 2 perts and with growth rates that are competitive with the principal bisymmetric mode. It can be argued that the galactic magnetic field not only facilitates star formation but may also bias the mass spectrum towards more massive stars, which interact much more violently with the interstellar medium. As an alternative to driving by a ‘grand design’ stellar density wave, a self-maintaining dynamo-wave may then point towards a ‘bootstrap’ picture: the energy supply for the interstellar turbulence is enhanced in regions of stronger magnetic field by magnetically catalysed star formation, and the bisymmetric magnetic field in tum is maintained by dynamo action due to the turbulence.

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