The Cosmic Baryon Cycle and Galaxy Mass Assembly in the FIRE Simulations
  We use cosmological simulations from the FIRE (Feedback In Realistic
Environments) project to study the baryon cycle and galaxy mass assembly for
central galaxies in the halo mass range $M_{\rm halo} \sim 10^{10} - 10^{13}
M_{\odot}$. By tracing cosmic inflows, galactic outflows, gas recycling, and
merger histories, we quantify the contribution of physically distinct sources
of material to galaxy growth. We show that in situ star formation fueled by
fresh accretion dominates the early growth of galaxies of all masses, while the
re-accretion of gas previously ejected in galactic winds often dominates the
gas supply for a large portion of every galaxy's evolution. Externally
processed material contributes increasingly to the growth of central galaxies
at lower redshifts. This includes stars formed ex situ and gas delivered by
mergers, as well as smooth intergalactic transfer of gas from other galaxies,
an important but previously under-appreciated growth mode. By $z=0$, wind
transfer, i.e. the exchange of gas between galaxies via winds, can dominate gas
accretion onto $\sim L^{*}$ galaxies over fresh accretion and standard wind
recycling. Galaxies of all masses re-accrete >50% of the gas ejected in winds
and recurrent recycling is common. The total mass deposited in the
intergalactic medium per unit stellar mass formed increases in lower mass
galaxies. Re-accretion of wind ejecta occurs over a broad range of timescales,
with median recycling times ($\sim 100-350$ Myr) shorter than previously found.
Wind recycling typically occurs at the scale radius of the halo, independent of
halo mass and redshift, suggesting a characteristic recycling zone around
galaxies that scales with the size of the inner halo and the galaxy's stellar
component.
