Magnetic Field Amplification in Young Galaxies
  The Universe at present is highly magnetized, with fields of the order of a
few 10^-5 G and coherence lengths larger than 10 kpc in typical galaxies like
the Milky Way. We propose that the magnetic field was amplified to this values
already during the formation and the early evolution of the galaxies.
Turbulence in young galaxies is driven by accretion as well as by supernova
(SN) explosions of the first generation of stars. The small-scale dynamo can
convert the turbulent kinetic energy into magnetic energy and amplify very weak
primordial magnetic seed fields on short timescales. The amplification takes
place in two phases: in the kinematic phase the magnetic field grows
exponentially, with the largest growth on the smallest non-resistive scale. In
the following non-linear phase the magnetic energy is shifted towards larger
scales until the dynamo saturates on the turbulent forcing scale. To describe
the amplification of the magnetic field quantitatively we model the
microphysics in the interstellar medium (ISM) of young galaxies and determine
the growth rate of the small-scale dynamo. We estimate the resulting saturation
field strengths and dynamo timescales for two turbulent forcing mechanisms:
accretion-driven turbulence and SN-driven turbulence. We compare them to the
field strength that is reached, when only stellar magnetic fields are
distributed by SN explosions. We find that the small-scale dynamo is much more
efficient in magnetizing the ISM of young galaxies. In the case of
accretion-driven turbulence a magnetic field strength of the order of 10^-6 G
is reached after a time of 24-270 Myr, while in SN-driven turbulence the dynamo
saturates at field strengths of typically 10^-5 G after only 4-15 Myr. This is
considerably shorter than the Hubble time. Our work can help to understand why
present-day galaxies are highly magnetized.
