Gas infall and radial transport in cosmological simulations of Milky
  Way-mass disks
  Observations indicate that a continuous supply of gas is needed to maintain
observed star formation rates in large, disky galaxies. To fuel star formation,
gas must reach the inner regions of such galaxies. Despite its crucial
importance for galaxy evolution, how and where gas joins galaxies is poorly
constrained observationally and is rarely explored in fully cosmological
simulations. To investigate gas accretion in the vicinity of galaxies, we
analyze the FIRE-2 cosmological zoom-in simulations for 4 Milky Way mass
galaxies (M_halo ~ 10E12 solar masses), focusing on simulations with cosmic ray
physics. We find that at z~0, gas approaches the disk with angular momentum
similar to the gaseous disk edge and low radial velocities, piling-up near the
edge and settling into full rotational support. Accreting gas moves
predominantly parallel to the disk with small but nonzero vertical velocity
components, and joins the disk largely in the outskirts as opposed to "raining"
down onto the disk. Once in the disk, gas trajectories are complex, being
dominated by spiral arm induced oscillations and feedback. However, time and
azimuthal averages show clear but slow net radial infall with transport speeds
of 1-3 km/s and net mass fluxes through the disk on the order of one solar mass
per year, comparable to the star formation rates of the galaxies and decreasing
towards galactic center as gas is sunk into star formation. These rates are
slightly higher in simulations without cosmic rays (1-7 km/s, ~4-5 solar masses
per year). We find overall consistency of our results with observational
constraints and discuss prospects of future observations of gas flows in and
around galaxies.
