Galaxy mergers on a moving mesh: a comparison with smoothed-particle
  hydrodynamics
  Galaxy mergers have been investigated for decades using smoothed particle
hydrodynamics (SPH), but recent work highlighting inaccuracies inherent in the
traditional SPH technique calls into question the reliability of previous
studies. We explore this issue by comparing a suite of Gadget-3 SPH simulations
of idealised (i.e., non-cosmological) isolated discs and galaxy mergers with
otherwise identical calculations performed using the moving-mesh code Arepo.
When black hole (BH) accretion and active galactic nucleus (AGN) feedback are
not included, the star formation histories (SFHs) obtained from the two codes
agree well. When BHs are included, the code- and resolution-dependent
variations in the SFHs are more significant, but the agreement is still good,
and the stellar mass formed over the course of a simulation is robust to
variations in the numerical method. During a merger, the gas morphology and
phase structure are initially similar prior to the starburst phase. However,
once a hot gaseous halo has formed from shock heating and AGN feedback (when
included), the agreement is less good. In particular, during the post-starburst
phase, the SPH simulations feature more prominent hot gaseous haloes and
spurious clumps, whereas with Arepo, gas clumps and filaments are less apparent
and the hot halo gas can cool more efficiently. We discuss the origin of these
differences and explain why the SPH technique yields trustworthy results for
some applications (such as the idealised isolated disc and galaxy merger
simulations presented here) but not others (e.g., gas flows onto galaxies in
cosmological hydrodynamical simulations).
