EDGE: A new approach to suppressing numerical diffusion in adaptive mesh
  simulations of galaxy formation
  We introduce a new method to mitigate numerical diffusion in adaptive mesh
refinement (AMR) simulations of cosmological galaxy formation, and study its
impact on a simulated dwarf galaxy as part of the 'EDGE' project. The target
galaxy has a maximum circular velocity of 21 km/s but evolves in a region which
is moving at up to 90 km/s relative to the hydrodynamic grid. In the absence of
any mitigation, diffusion softens the filaments feeding our galaxy. As a
result, gas is unphysically held in the circumgalactic medium around the galaxy
for 320 Myr, delaying the onset of star formation until cooling and collapse
eventually triggers an initial starburst at z=9. Using genetic modification, we
produce 'velocity-zeroed' initial conditions in which the grid-relative
streaming is strongly suppressed; by design, the change does not significantly
modify the large scale structure or dark matter accretion history. The
resulting simulation recovers a more physical, gradual onset of star formation
starting at z=17. While the final stellar masses are nearly consistent ($4.8
\times 10^6\,M_\odot$ and $4.4\times 10^6\,M_\odot$ for unmodified and
velocity-zeroed respectively), the dynamical and morphological structure of the
z=0 dwarf galaxies are markedly different due to the contrasting histories. Our
approach to diffusion suppression is suitable for any AMR zoom cosmological
galaxy formation simulations, and is especially recommended for those of small
galaxies at high redshift.
