The equilibrium view on dust and metals in galaxies: Galactic outflows
  drive low dust-to-metal ratios in dwarf galaxies
  Most galaxy evolution simulations as well as a variety of observational
methods assume a linear scaling between the (galaxy-averaged) dust-to-gas ratio
D and metallicity Z of the interstellar medium (ISM). Indeed, nearby galaxies
with solar or moderately sub-solar metallicities clearly follow this trend
albeit with significant scatter. However, a growing number of observations show
that the linear scaling breaks down for metal-poor galaxies (Z<0.2 Z_sun),
highlighting the need for a more sophisticated modeling of the dust-to-metal
ratio of galaxies. Here we study the co-evolution of dust and metal abundances
in galaxies with the help of a dynamical, one-zone model that incorporates dust
formation and destruction processes in addition to gas inflows, outflows, and
metal enrichment. The dynamical model is consistent with various observational
constraints, including the stellar mass -- metallicity relation, the stellar
mass -- halo mass relation, and the observed Z -- D relation for both
metal-poor and metal-rich galaxies. The functional form of the Z -- D relation
follows from a basic equilibrium ansatz, similar to the ideas used previously
to model the stellar mass -- metallicity relation. Galactic outflows regulate
the inflow rate of gas from the cosmic web for galaxies of a given star
formation rate. The mass loading factor of outflows thus dictates the rate at
which the dust and metal content of the ISM is diluted. The stellar mass
dependence of the mass loading factor drives the evolution of metallicities,
dust-to-gas ratios, and dust-to-metal ratios in galaxies.
