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−1 but evolves in a region that is moving at up to 90 km s−1 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 × 106 M and 4.4 × 106 M 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 redshif
