Astron. Astrophys. 358, 1142-1150 (2000)

Accretion and galactic dynamos

D. Moss ¹, A. Shukurov ² and D. Sokoloff <sup>2,3

1</sup> Department of Mathematics, University of Manchester, Manchester M13 9PL, UK
² Department of Mathematics, University of Newcastle, Newcastle on Tyne NE1 7RU, UK
³ Physics Department, Moscow University, Moscow 19899, Russia

Received 5 January 2000 / Accepted 26 April 2000

Abstract

We show that radial flows (directed either inward or outward) suppress mean-field dynamo action in a thin disc. Under conditions typical of spiral galaxies, the reduction in the growth rate of the large-scale magnetic field can reach 25-30%. The dynamo-generated galactic magnetic fields themselves can drive radial flows at a rate of order . These radial flows can contribute to the saturation of the dynamo action together with other nonlinear effects. We also present arguments, and an illustrative computation, to support the view that in the inner regions of barred galaxies the magnetically driven mass accretion rate may be substantially larger than the above estimate due to deviations of magnetic field from axial symmetry. The magnetically driven inflow rate in barred galaxies can reach about .

Key words: accretion, accretion disks magnetic fields ISM: magnetic fields galaxies: magnetic fields

Send offprint requests to: D. Moss

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Contents

• 1. Introduction
• 2. Radial flows in spiral galaxies
• 3. The effect of accretion flows on disc dynamos
• 4. A simplified numerical model
• 5. Magnetically driven accretion flows
  • 5.1. The magnetically driven accretion flow
  • 5.2. The accretion rate
• 6. Discussion
• Acknowledgements
• Appendices
  • A: expansion in free decay modes for the local dynamo equations
• References

© European Southern Observatory (ESO) 2000

Online publication: June 20, 2000
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