  Recent theoretical studies have shown that galaxies at high redshift are fed
by cold, dense gas filaments, suggesting angular momentum transport by gas
differs from that by dark matter. Revisiting this issue using high-resolution
cosmological hydrodynamics simulations with adaptive mesh refinement, we find
that at the time of accretion, gas and dark matter do carry a similar amount of
specific angular momentum, but that it is systematically higher than that of
the dark matter halo as a whole. At high redshift, freshly accreted gas rapidly
streams into the central region of the halo, directly depositing this large
amount of angular momentum within a sphere of radius r=0.1rvir. In contrast,
dark matter particles pass through the central region unscathed, and a fraction
of them ends up populating the outer regions of the halo (r/rvir>0.1),
redistributing angular momentum in the process. As a result, large-scale
motions of the cosmic web have to be considered as the origin of gas angular
momentum rather than its virialised dark matter halo host. This generic result
holds for halos of all masses at all redshifts, as radiative cooling ensures
that a significant fraction of baryons remain trapped at the centre of the
halos. Despite this injection of angular momentum enriched gas, we predict an
amount for stellar discs which is in fair agreement with observations at z=0.
This arises because the total specific angular momentum of the baryons remains
close to that of dark matter halos. We propose a new scenario where gas
efficiently carries the angular momentum generated by large-scale structure
motions deep inside dark matter halos, redistributing it only in the vicinity
of the disc.
