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Astron. Astrophys. 353, 108-116 (2000)


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The role of Kelvin-Helmholtz modes in superwinds of primeval galaxies for the magnetization of the intergalactic medium

G.T. Birk 1, H. Wiechen 1, H. Lesch 1 and P.P. Kronberg 2

1 Institut für Astronomie und Astrophysik der Universität München, Scheinerstrasse 1, 81679 München, Germany
2 Department of Physics, University of Toronto, 60 St George, Toronto, M5S 1A7, Canada

Received 30 July 1999 / Accepted 26 October 1999

Abstract

Superwinds of primeval dwarf galaxies seem to play an important role in the early magnetization of the intergalactic medium. Realistic estimates of the "ultimate", post-outflow diffuse intergalactic magnetic field strengths cannot be made with simple flux conservation arguments. Rather one has to consider dynamical field regeneration mechanisms. Furthermore the role of the neutral component of the superwind outflow gas needs to be taken into account since local-universe outflow winds are known to be only partially ionized.

Kelvin-Helmholtz instabilities are one plausible candidate for the amplification of magnetic flux expelled from the starbursting primeval galaxies. We examine solutions of the analytically derived generalized dispersion relation that is valid for partially ionized plasmas. Our calculations indicate that Kelvin-Helmholtz modes can operate fast enough to amplify the magnetic field strengths in superwinds within the timescale of the outflow.

Plasma-neutral gas simulations show that the non-linear evolution of the Kelvin-Helmholtz modes resemble the pure MHD one and corroborate the idea that the coupling of the neutral and ionized components is sufficient to convert the free energy supplied by the superwinds to magnetic field energy.

We argue that further amplification can be expected due to the development of smaller vortices and sharper magnetic field gradients during the continuous injection of magnetic flux.

Key words: galaxies: magnetic fields – galaxies: intergalactic medium – galaxies: dwarf – galaxies: starburst – instabilities – plasmas

Send offprint requests to: G.T. Birk (birk@usm.uni-muenchen.de)

© European Southern Observatory (ESO) 2000

Online publication: December 8, 1999

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