Metal diffusion in smoothed particle hydrodynamics simulations of dwarf
  galaxies
  We perform a series of smoothed particle hydrodynamics simulations of
isolated dwarf galaxies to compare different metal mixing models. In
particular, we examine the role of diffusion in the production of enriched
outflows, and in determining the metallicity distributions of gas and stars. We
investigate different diffusion strengths, by changing the pre-factor of the
diffusion coefficient, by varying how the diffusion coefficient is calculated
from the local velocity distribution, and by varying whether the speed of sound
is included as a velocity term. Stronger diffusion produces a tighter
[O/Fe]-[Fe/H] distribution in the gas, and cuts off the gas metallicity
distribution function at lower metallicities. Diffusion suppresses the
formation of low-metallicity stars, even with weak diffusion, and also strips
metals from enriched outflows. This produces a remarkably tight correlation
between "metal mass-loading" (mean metal outflow rate divided by mean metal
production rate) and the strength of diffusion, even when the diffusion
coefficient is calculated in different ways. The effectiveness of outflows at
removing metals from dwarf galaxies and the metal distribution of the gas is
thus dependent on the strength of diffusion. By contrast, we show that the
metallicities of stars are not strongly dependent on the strength of diffusion,
provided that some diffusion is present.
