Tracing Outflowing Metals in Simulations of Dwarf and Spiral Galaxies
  We analyze the metal accumulation in dwarf and spiral galaxies by following
the history of metal enrichment and outflows in a suite of twenty
high-resolution simulated galaxies. These simulations agree with the observed
stellar and gas-phase mass-metallicity relation, an agreement that relies on
large fractions of the produced metals escaping into the CGM. For instance, in
galaxies with Mvir ~ 1e9.5 -- 1e10 solar masses, we find that about ~ 85% of
the available metals are outside of the galactic disk at z = 0, although the
fraction decreases to a little less than half in Milky Way-mass galaxies. In
many cases, these metals are spread far beyond the virial radius. We analyze
the metal deficit within the ISM and stars in the context of previous work
tracking the inflow and outflow of baryons. Outflows are prevalent across the
entire mass range, as is reaccretion. We find that between 40 and 80% of all
metals removed from the galactic disk are later reaccreted. The outflows
themselves are metal enriched relative to the ISM by a factor of 0.2 dex
because of the correspondence between sites of metal enrichment and outflows.
As a result, the metal mass loading factor scales as eta_metals \propto
v_circ^-0.91, a somewhat shallower scaling than the total mass loading factor.
We analyze the simulated galaxies within the context of analytic chemical
evolution models by determining their net metal expulsion efficiencies, which
encapsulate the rates of metal loss and reaccretion. We discuss these results
in light of the inflow and outflow properties necessary for reproducing the
mass-metallicity relation.
