How Gas Accretion Feeds Galactic Disks
  Numerous observations indicate that galaxies need a continuous gas supply to
fuel star formation and explain the star formation history. However, direct
observational evidence of gas accretion remains rare. Using the EAGLE
cosmological hydrodynamic simulation suite, we study cold gas accretion onto
galaxies and the observational signatures of the cold gas kinematics. For EAGLE
galaxies at z=0.27, we find that cold gas accretes onto galaxies
anisotropically with typical inflow speeds between 20 km s$^{-1}$ and 60 km
s$^{-1}$. Most of these galaxies have comparable mass inflow rates and star
formation rates, implying that the cold inflowing gas plausibly accounts for
sustaining the star-forming activities of the galaxies. As motivation for
future work to compare the cold gas kinematics with measurements from quasar
sightline observations, we select an EAGLE galaxy with an extended cold gas
disk, and we probe the cold gas using mock quasar sightlines. We demonstrate
that by viewing the disk edge-on, sightlines at azimuthal angles below
10$^{\circ}$ and impact parameters out to 60 pkpc can detect cold gas that
corotates with the galaxy disk. This example suggests cold gas disks that
extend beyond the optical disks possibly explain the sightline observations
that detect corotating cold gas near galaxy major axes.
