  A long standing problem for hierarchical disk galaxy formation models has
been the simultaneous matching of the zero point of the Tully-Fisher relation
and the galaxy luminosity function (LF). We illustrate this problem for a
typical disk galaxy and discuss three solutions: low stellar mass-to-light
ratios, low initial dark halo concentrations, and no halo contraction. We
speculate that halo contraction may be reversed through a combination of mass
ejection through feedback and angular momentum exchange brought about by
dynamical friction between baryons and dark matter during the disk formation
process.
