Origin of the magnetic spiral arms in the galaxy NGC6946

Abstract

THE recent discovery¹ of coherent, large-scale magnetic spiral arms located between the optical spiral arms of the nearby galaxy NGC6946 is difficult to reconcile with current galactic dynamo theory². The formation of optical spiral arms is generally attributed to the role of density waves in the gas of the galactic disk, which trigger the formation of massive, luminous stars; turbulent processes in the gas are also believed to play a key role in the generation of the galactic magnetic field. Here we present the results of magnetohydrodynamical (MHD) calculations, which show that both fast (∼20km s⁻¹) and slow (∼0.2 km s⁻¹) MHD density waves can exist in a rotating galactic disk. Slow MHD density waves occur over the portion of a galactic disk that rotates almost rigidly³, and can give rise to a pattern of polarized radio emission similar to that observed in NGC6946, where the magnetic field is strong between the optical spiral arms. In contrast, fast MHD density waves produce spiral magnetic structures that are roughly coincident with the optical spiral arms—and which extend into the differentially rotating disk—as observed in the Whirlpool galaxy, NGC5194 (refs 4–6).