The Angular Momentum Problem in Cosmological Simulations of Disk Galaxy
  Formation
  We conduct a systematic study of the angular momentum problem in numerical
simulations of disk galaxy formation. We investigate the role of numerical
resolution using a semi-cosmological setup which combines an efficient use of
the number of particles in an isolated halo while preserving the hierarchical
build-up of the disk through the merging of clumps. We perform the same
simulation varying the resolution over 4 orders of magnitude. Independent on
the level of resolution, the loss of angular momentum stays the same and can be
tied to dynamical friction during the build-up phase. This is confirmed in a
cosmological simulation. We also perform simulations including star formation
and star formation and supernova feedback. While the former has no influence on
the angular momentum problem, the latter reduces the loss to a level
potentially in agreement with observations. This is achieved through a
suppression of early star formation and therefore the formation of a large,
slowly rotating bulge. We conclude that feedback is a critical component to
achieve realistic disk galaxies in cosmological simulations. Numerical
resolution is important, but by itself not capable of solving the angular
momentum problem.
