Zusammenfassung
Astrophysical dynamo theories provide various mechanisms for magnetic field
amplification inside galaxies, where weak initial fields grow exponentially on
various timescales. We investigate the particular role played by stellar
feedback mechanisms in creating strong fluid turbulence, allowing for a
magnetic dynamo to emerge. Performing magnetohydrodynamic simulations of
isolated cooling halos, for both dwarf and Milky Way sized objects, we compare
the magnetic field evolution for various initial field topologies and various
stellar feedback mechanisms. We find that feedback can indeed drive strong gas
turbulence which gives rise to a fast exponential magnetic field growth. Our
simulations feature typical properties of Kolmogorov turbulence with a \$k
^-5/3\$ kinetic energy spectrum, as well as the characteristic properties of a
small-scale dynamo, with a \$k^3/2\$ magnetic energy spectrum as predicted by
Kazantsev dynamo theory. In these feedback-dominated galaxies, stellar feedback
provides forcing on large scales close to the halo scale radius, providing thus
exponential field growth on all scales within the galaxy. We also investigate
simulations with a final quiescent phase by manually turning off the feedback.
As turbulence decreases, the galactic fountain settles into a thin,
rotationally supported disk. The magnetic field develops a large-scale,
well-ordered structure with quadrupole symmetry, irrespective of the initial
field topology, which is in good agreement with magnetic field observations of
nearby spirals. Our findings suggest that weak initial seed fields were first
amplified by a small-scale dynamo during a violent, feedback-dominated early
phase in the galaxy formation history, followed by a more quiescent evolution,
where the fields have slowly decayed or were maintained via large-scale dynamo
action.
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