Dust diffusion in SPH simulations of an isolated galaxy
  We compute the evolution of the grain size distribution (GSD) in a suite of
numerical simulations of an isolated Milky-Way-like galaxy using the
$N$-body/smoothed-particle-hydrodynamics code {\sc Gadget-4}. The full GSD is
sampled on a logarithmically spaced grid with 30 bins, and its evolution is
calculated self-consistently with the hydrodynamical and chemical evolution of
the galaxy using a state-of-the-art star formation and feedback model. In
previous versions of this model, the GSD tended to be slightly biased towards
larger grains and the extinction curve had a tendency to be flatter than the
observations. This work addresses these issues by considering the diffusion of
dust and metals through turbulence on subgrid scales and introducing a
multi-phase subgrid model that enables a smoother transition from diffuse to
dense gas. We show that diffusion can significantly enhance the production of
small grains and improve the agreement with the observed dust extinction curve
in the Milky Way.
