Global density and magnetic field structures in spiral galaxies: spiral structure and bisymmetric magnetic field

Abstract

Spiral galaxies exhibit global structure both in their neutral density distribution as well as in magnetic field distribution. The former is now known to be related to the spiral structure of these galaxies. However, some problems relating to the nature of the spiral patterns remain, namely, what determines the nature of the patterns, "leading" vs. "trailing" patterns. For the magnetic field structure, on the other hand, there is as yet no satisfactory theory to explain its growth and maintenance, the "kinematic dynamo theory" having been shown recently to have failed. We give here theories for both the density wave in the neutral disk as well as for the bisymmetric magnetic field. We show that the temperature (velocity dispersion) of the disk plays a crucial role in determining the (observed) "trailing" pattern. For the magnetic field bisymmetric form we invoke the inverted density distribution of the H_II regions (found in most galaxy types close to the disk centre) so that the Rayleigh-Taylor instability of the inverted distribution supplies the free energy required for the growth of the bisymmetric magnetic field spirals.