The impact of magnetic fields on cosmological galaxy mergers. I:
  Reshaping gas and stellar discs
  Mergers play an important role in galaxy evolution. In particular, major
mergers are able to have a transformative effect on galaxy morphology. In this
paper, we investigate the role of magnetic fields in gas-rich major mergers. To
this end, we run a series of high-resolution magnetohydrodynamic (MHD) zoom-in
simulations with the moving-mesh code Arepo and compare the outcome with
hydrodynamic simulations run from the same initial conditions. This is the
first time that the effect of magnetic fields in major mergers has been
investigated in a cosmologically-consistent manner. In contrast to previous
non-cosmological simulations, we find that the inclusion of magnetic fields has
a substantial impact on the production of the merger remnant. Whilst magnetic
fields do not strongly affect global properties, such as the star formation
history, they are able to significantly influence structural properties.
Indeed, MHD simulations consistently form remnants with extended discs and
well-developed spiral structure, whilst hydrodynamic simulations form more
compact remnants that display distinctive ring morphology. We support this work
with a resolution study and show that whilst global properties are broadly
converged across resolution and physics models, morphological differences only
develop given sufficient resolution. We argue that this is due to the more
efficient excitement of a small-scale dynamo in higher resolution simulations,
resulting in a more strongly amplified field that is better able to influence
gas dynamics.
