Recycled stellar ejecta as fuel for star formation and implications for
  the origin of the galaxy mass-metallicity relation
  We use cosmological, hydrodynamical simulations from the EAGLE and OWLS
projects to assess the significance of recycled stellar ejecta as fuel for star
formation. The fractional contributions of stellar mass loss to the cosmic star
formation rate (SFR) and stellar mass densities increase with time, reaching
$35 \%$ and $19 \%$, respectively, at $z=0$. The importance of recycling
increases steeply with galaxy stellar mass for $M_{\ast} < 10^{10.5}$
M$_{\odot}$, and decreases mildly at higher mass. This trend arises from the
mass dependence of feedback associated with star formation and AGN, which
preferentially suppresses star formation fuelled by recycling. Recycling is
more important for satellites than centrals and its contribution decreases with
galactocentric radius. The relative contribution of AGB stars increases with
time and towards galaxy centers. This is a consequence of the more gradual
release of AGB ejecta compared to that of massive stars, and the preferential
removal of the latter by star formation-driven outflows and by lock up in
stellar remnants. Recycling-fuelled star formation exhibits a tight, positive
correlation with galaxy metallicity, with a secondary dependence on the
relative abundance of alpha elements (which are predominantly synthesized in
massive stars), that is insensitive to the subgrid models for feedback. Hence,
our conclusions are directly relevant for the origin of the mass-metallicity
relation and metallicity gradients. Applying the relation between recycling and
metallicity to the observed mass-metallicity relation yields our best estimate
of the mass-dependent contribution of recycling. For centrals with a mass
similar to that of the Milky Way, we infer the contributions of recycled
stellar ejecta to the SFR and stellar mass to be $35 \%$ and $20 \%$,
respectively.
