ORIGIN OF THE MAGNETIC SPIRAL ARMS IN THE GALAXY NGC6946

THE recent discovery(1) of coherent, large-scale magnetic spiral arms located between the optical spiral arms of the nearby galaxy NGC6946 i s difficult to reconcile with current galactic dynamo theory(2), The f ormation of optical spiral arms is generally attributed to the role of density waves in the gas of the galactic disk, which trigger the form ation of massive, luminous stars; turbulent processes in the gas are a lso believed to play a key role in the generation of the galactic magn etic field, Here we present the results of magnetohydrodynamical (MHD) calculations, which show that both fast (similar to 20 km s(-1)) and slow (similar to 0.2 km s(-1)) MHD density waves can exist in a rotati ng galactic disk, Slow MHD density waves occur over the portion of a g alactic disk that rotates almost rigidly(3), and can give rise to a pa ttern of polarized radio emission similar to that observed in NGC6946, where the magnetic field is strong between the optical spiral arms. I n contrast, fast MHD density waves produce spiral magnetic structures that are roughly coincident with the optical spiral arms-and which ext end into the differentially rotating disk-as observed in the Whirlpool galaxy, NGC5194 (refs 4-6).