Cosmological baryon transfer in the SIMBA simulations
  We present a framework for characterizing the large scale movement of baryons
relative to dark matter in cosmological simulations, requiring only the initial
conditions and final state of the simulation. This is performed using the
spread metric which quantifies the distance in the final conditions between
initially neighbouring particles, and by analysing the baryonic content of
final haloes relative to that of the initial Lagrangian regions defined by
their dark matter component. Applying this framework to the SIMBA cosmological
simulations, we show that 40% (10%) of cosmological baryons have moved $>
1h^{-1}~ {\rm Mpc}^{-1}$ ($3h^{-1}~ {\rm Mpc}^{-1}$) by $z=0$, due primarily to
entrainment of gas by jets powered by AGN, with baryons moving up to $12h^{-1}~
{\rm Mpc}^{-1}$ away in extreme cases. Baryons decouple from the dynamics of
the dark matter component due to hydrodynamic forces, radiative cooling, and
feedback processes. As a result, only 60% of the gas content in a given halo at
$z=0$ originates from its Lagrangian region, roughly independent of halo mass.
A typical halo in the mass range $M_{\rm vir} = 10^{12}$--$10^{13}{\rm
M}_\odot$ only retains 20% of the gas originally contained in its Lagrangian
region. We show that up to 20% of the gas content in a typical Milky Way mass
halo may originate in the region defined by the dark matter of another halo.
This inter-Lagrangian baryon transfer may have important implications for the
origin of gas and metals in the circumgalactic medium of galaxies, as well as
for semi-analytic models of galaxy formation and "zoom-in" simulations.
