A small-scale dynamo in feedback-dominated galaxies - III. Cosmological
  simulations
  Magnetic fields are widely observed in the Universe in virtually all
astrophysical objects, from individual stars to entire galaxies, even in the
intergalactic medium, but their specific generation has long been debated. Due
to the development of more realistic models of galaxy formation, viable
scenarios are emerging to explain cosmic magnetism, thanks to both deeper
observations and more efficient and accurate computer simulations. We present
here a new cosmological high-resolution zoom-in magnetohydrodynamic (MHD)
simulation, using the adaptive mesh refinement (AMR) technique, of a dwarf
galaxy with an initially weak and uniform magnetic seed field that is amplified
by a small-scale dynamo driven by supernova-induced turbulence. As first
structures form from the gravitational collapse of small density fluctuations,
the frozen-in magnetic field separates from the cosmic expansion and grows
through compression. In a second step, star formation sets in and establishes a
strong galactic fountain, self-regulated by supernova explosions. Inside the
galaxy, the interstellar medium becomes highly turbulent, dominated by strong
supersonic shocks, as demonstrated by the spectral analysis of the gas kinetic
energy. In this turbulent environment, the magnetic field is quickly amplified
via a small-scale dynamo process and is finally carried out into the
circumgalactic medium by a galactic wind. This realistic cosmological
simulation explains how initially weak magnetic seed fields can be amplified
quickly in early, feedback-dominated galaxies, and predicts, as a consequence
of the small scale dynamo process, that high-redshift magnetic fields are
likely to be dominated by their small scale components.
