Vertical magnetic fields above the discs of spiral galaxies.
Abstract
We investigate the magnetic fields above the discs of spiral galaxies in the framework of axisymmetric nonlinear mean-field dynamo models for a disc surrounded by a spherical halo, using realistic rotation curves. We consider, in particular, NGC 4631 and NGC 891, and include turbulent diamagnetism, an anisotropy of the α effect and a galactic wind. In these model magnetic field is generated in a disc of scale height 1.5 kpc and distorted by the wind in the halo. For typical wind velocities of 50.. .200 km/s in the halo we find good qualitative agreement between the observed polarisation maps and those synthesised from the magnetic fields of our models: Poloidal synthesised fields dominate in the halo of NGC 4631, and horizontal fields in that of NGC 891. In some cases, a galactic wind can enhance dynamo action, contrary to previous expectations. Our results indicate that turbulent diamagnetism plays an important role in galactic dynamos.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- April 1993
- Bibcode:
- 1993A&A...271...36B