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Small-scale dynamo action in primordial halos

Published online by Cambridge University Press:  18 July 2013

Jennifer Schober ,
Dominik R. G. Schleicher ,
Ralf S. Klessen ,
Christoph Federrath ,
Stefano Bovino ,
Simon Glover  and
Robi Banerjee
Jennifer Schober
Affiliation:
Heidelberg University, Center for Astronomy, Institute for Theoretical Astrophysics, Albert-Ueberle-Str 2, 69120 Heidelberg, Germany
Dominik R. G. Schleicher
Affiliation:
Göttingen University, Institute for Astrophysics, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
Ralf S. Klessen
Affiliation:
Heidelberg University, Center for Astronomy, Institute for Theoretical Astrophysics, Albert-Ueberle-Str 2, 69120 Heidelberg, Germany
Christoph Federrath
Affiliation:
Monash University, School of Mathematical Sciences, Monash Centre for Astrophysics, Clayton Vic 3800, Australia
Stefano Bovino
Affiliation:
Göttingen University, Institute for Astrophysics, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
Simon Glover
Affiliation:
Heidelberg University, Center for Astronomy, Institute for Theoretical Astrophysics, Albert-Ueberle-Str 2, 69120 Heidelberg, Germany
Robi Banerjee
Affiliation:
Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
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Abstract

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The first galaxies form due to gravitational collapse of primordial halos. During this collapse, weak magnetic seed fields get amplified exponentially by the small-scale dynamo - a process converting kinetic energy from turbulence into magnetic energy. We use the Kazantsev theory, which describes the small-scale dynamo analytically, to study magnetic field amplification for different turbulent velocity correlation functions. For incompressible turbulence (Kolmogorov turbulence), we find that the growth rate is proportional to the square root of the hydrodynamic Reynolds number, Re1/2. In the case of highly compressible turbulence (Burgers turbulence) the growth rate increases proportional to Re1/3. With a detailed chemical network we are able to follow the chemical evolution and determine the kinetic and magnetic viscosities (due to Ohmic and ambipolar diffusion) during the collapse of the halo. This way, we can calculate the growth rate of the small-scale dynamo quantitatively and predict the evolution of the small-scale magnetic field. As the magnetic energy is transported to larger scales on the local eddy-timescale, we obtain an estimate for the magnetic field on the Jeans scale. Even there, we find that equipartition with the kinetic energy is reached on small timescales. Dynamically relevant field structures can thus be expected already during the formation of the first objects in the Universe.

Keywords

ISM: magnetic fieldsMHDstars: formationearly universe
Type
Contributed Papers
Information
Proceedings of the International Astronomical Union , Volume 8 , Symposium S294: Solar and Astrophysical Dynamos and Magnetic Activity , August 2012 , pp. 237 - 248
Copyright
Copyright © International Astronomical Union 2013 

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