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Galactic Dynamics and Magnetic Field Amplification

Published online by Cambridge University Press:  19 July 2016

Harald Lesch
Harald Lesch*
Affiliation:
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, W-5300 Bonn 1, FRG

Abstract

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Stimulated by recent high frequency radio polarization measurements of M83 and M51, we consider the influence of non-axisymmetric features (bars, spiral arms, etc…) on galactic magnetic fields. The time scale for the field amplification due to the non-axisymmetric velocity field is related to the time scale of angular momentum transport in the disk by the non-axisymmetric features. Due to its dissipational character (cooling and angular momentum transport) the gas plays a major role for the excitation of non-axisymmetric instabilities. Since it is the gaseous component of the interstellar gas in which magnetic field amplification takes place we consider the interplay of gasdynamical processes triggered by gravitational instabilities and magnetic fields. A comparison with the time scale for dynamo action in a disk from numerical models for disk dynamos gives the result that field amplification by non-axisymmetric features is faster in galaxies like M83 (strong bar) and M51 (compagnion and very distinct spiral structure), than amplification by an axisymmetric dynamo. Furthermore, we propose that axisymmetric gravitational instabilities may provide the turbulent magnetic diffusivity ηT. Based on standard galaxy models we obtain a radially dependent diffusivity whose numerical value rises from 1025cm 2s −1 to 1027cm 2s −1, declining for large radii.

Type
9. Magnetohydrodynamic Phenomena in the Interstellar Medium Related to Dynamo Processes
Information
Symposium - International Astronomical Union , Volume 157: The Cosmic Dynamo , 1993 , pp. 395 - 401
Copyright
Copyright © Kluwer 1993 

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