Galactic inflow and wind recycling rates in the EAGLE simulations
  The role of galactic wind recycling represents one of the largest unknowns in
galaxy evolution, as any contribution of recycling to galaxy growth is largely
degenerate with the inflow rates of first-time infalling material, and the
rates with which outflowing gas and metals are driven from galaxies. We present
measurements of the efficiency of wind recycling from the EAGLE cosmological
simulation project, leveraging the statistical power of large-volume
simulations that reproduce a realistic galaxy population. We study wind
recycling at the halo scale, i.e. gas that has been ejected beyond the halo
virial radius, and at the galaxy scale, i.e. gas that has been ejected from the
ISM to at least $\approx 10 \, \%$ of the virial radius (thus excluding
smaller-scale galactic fountains). Galaxy-scale wind recycling is generally
inefficient, with a characteristic return timescale that is comparable or
longer than a Hubble time, and with an efficiency that clearly peaks at the
characteristic halo mass of $M_{200} = 10^{12} \, \mathrm{M_\odot}$.
Correspondingly, the majority of gas being accreted onto galaxies in EAGLE is
infalling for the first time. At the halo scale, the efficiency of recycling
onto haloes differs by orders of magnitude from values assumed by semi-analytic
galaxy formation models. Differences in the efficiency of wind recycling with
other hydrodynamical simulations are currently difficult to assess, but are
likely smaller. We are able to show that the fractional contribution of wind
recycling to galaxy growth is smaller in EAGLE than in some other simulations.
We find that cumulative first-time gas accretion rates at the virial radius are
reduced relative to the expectation from dark matter accretion for haloes with
mass, $M_{200} < 10^{12} \, \mathrm{M_\odot}$, indicating efficient
preventative feedback on halo scales.
