  We investigate the evolution of angular momentum in simulations of galaxy
formation in a cold dark matter universe. We analyse two model galaxies
generated in the N-body/hydrodynamic simulations of Okamoto et al. Starting
from identical initial conditions, but using different assumptions for the
baryonic physics, one of the simulations produced a bulge-dominated galaxy and
the other one a disc-dominated galaxy. The main difference is the treatment of
star formation and feedback, both of which were designed to be more efficient
in the disc-dominated object. We find that the specific angular momentum of the
disc-dominated galaxy tracks the evolution of the angular momentum of the dark
matter halo very closely: the angular momentum grows as predicted by linear
theory until the epoch of maximum expansion and remains constant thereafter. By
contrast, the evolution of the angular momentum of the bulge-dominated galaxy
resembles that of the central, most bound halo material: it also grows at first
according to linear theory, but 90% of it is rapidly lost as pre-galactic
fragments, into which gas had cooled efficiently, merge, transferring their
orbital angular momentum to the outer halo by tidal effects. The disc-dominated
galaxy avoids this fate because the strong feedback reheats the gas which
accumulates in an extended hot reservoir and only begins to cool once the
merging activity has subsided. Our analysis lends strong support to the
classical theory of disc formation whereby tidally torqued gas is accreted into
the centre of the halo conserving its angular momentum.
