Simulating Gas Inflow at the Disk-Halo Interface
  The interaction between inflowing gas clouds and galactic outflows at the
interface where the galactic disk transitions into the circumgalactic medium is
an important process in galaxy fueling, yet remains poorly understood. Using a
series of tall-box hydrodynamic ENZO simulations, we have studied the
interaction between smooth gas inflow and supernovae-driven outflow at the
disk-halo interface with pc-scale resolution. A realistic wind of outflowing
material is generated by supernovae explosions in the disk, while inflowing gas
is injected at the top boundary of the simulation box with an injection
velocity ranging from $10-100 \rm \ km \ s^{-1}$. We find that cooling and
hydrodynamic instabilities drive the injected gas to fragment into cold ($\sim
10^{3}$ K) cloud clumps with typical densities of $\sim 1 \rm \ cm^{-3}$. These
clumps initially accelerate before interacting and partially mixing with the
outflow and decelerating to velocities in the 50-100 $\rm km \ s^{-1}$ range.
When the gas clumps hit the disk, $10\%-50 \%$ of the injected material is able
to accrete (depending on the injection velocity). Clumps originating from gas
injected with a higher initial velocity approach the disk with greater ram
pressure, allowing them to penetrate through the disk in low density regions.
We use (equilibrium) CLOUDY photoionization models to generate absorption and
emission signatures of gas accretion, finding that our mock HI and H$\alpha$
observables are prominent and generally consistent with measurements in the
Milky Way. We do not predict enhanced emission/absorption for higher ionization
states such as OVI.
