Galactic m\'enage \`a trois: Simulating magnetic fields in colliding
  galaxies
  We present high resolution simulations of a multiple merger of three disk
galaxies including the evolution of magnetic fields performed with the
N-body/SPH code Gadget. For the first time, we embed the galaxies in a
magnetized, low-density medium, thus modeling an ambient IGM. The simulations
include radiative cooling and a model for star formation and supernova
feedback. The progenitor disks have initial magnetic seed fields in the range
of 10e-9 to 10e-6 G and the IGM has initial fields of 10e-12 to 10e-9 G. The
simulations are compared to a run excluding magnetic fields. We show that the
propagation of interaction-driven shocks depends significantly on the initial
magnetic field strength. The shocks propagate faster in simulations with
stronger initial field, suggesting that the shocks are supported by magnetic
pressure. The Mach numbers of the shocks range from approximately M=1.5 for the
non-magnetized case up to M=6 for the highest initial magnetization, resulting
in higher temperatures of the shock heated IGM gas. The magnetic field in the
system saturates rapidly after the mergers at ~ 10e-6 G within the galaxies and
~ 10e-8 G in the IGM independent of the initial value. These field strengths
agree with observed values and correspond to the equipartition value of the
magnetic pressure with the turbulent pressure in the system. We also present
synthetic radio and polarization maps for different phases of the evolution
showing that shocks driven by the interaction produce a high amount of
polarized emission. These idealized simulations indicate that magnetic fields
play an important role for the hydrodynamics of the IGM during galactic
interactions. We also show that even weak seed fields are efficiently
strengthened during multiple galactic mergers. This interaction driven
amplification might have been a key process for the magnetization of the
Universe.
