High Angular Momentum Halo Gas: a Feedback and Code-Independent
  Prediction of LCDM
  We investigate angular momentum acquisition in Milky Way-sized galaxies by
comparing five high resolution zoom-in simulations, each implementing identical
cosmological initial conditions, but utilizing different hydrodynamic codes:
Enzo, Art, Ramses, Arepo, and Gizmo-PSPH. Each code implements a distinct set
of feedback and star formation prescriptions. We find that while many galaxy
and halo properties vary between the different codes (and feedback
prescriptions), there is qualitative agreement on the process of angular
momentum acquisition in the galaxy's halo. In all simulations, cold filamentary
gas accretion to the halo results in ~4 times more specific angular momentum in
cold halo gas ($\lambda_{cold} \gtrsim 0.1$) than in the dark matter halo. At
z>1, this inflow takes the form of inspiraling cold streams that are
co-directional in the halo of the galaxy and are fueled, aligned, and
kinematically connected to filamentary gas infall along the cosmic web. Due to
the qualitative agreement among disparate simulations, we conclude that the
buildup of high angular momentum halo gas and the presence of these inspiraling
cold streams are robust predictions of Lambda Cold Dark Matter galaxy
formation, though the detailed morphology of these streams is significantly
less certain. A growing body of observational evidence suggests that this
process is borne out in the real universe.
