Formation of disk galaxies: Feedback and the angular momentum problem

When only cooling processes are included, smoothed-particle hydrodynamical (SPH) simulations of galaxy formation in a cold dark matter hierarchical cl ustering scenario consistently produce collapsed objects that are deficient in angular momentum by a factor of about 25 relative to the disks of obser ved spiral galaxies. It is widely hoped that proper allowance for star form ation feedback effects will resolve this discrepancy. We explore and compar e the effects of including two different types of feedback event: uniform r eheating of the entire universe to 5 x 10(5) K at a redshift of z similar t o 6, and gas blowout from pregalactic gas clouds comparable to present-day dwarf galaxies at a somewhat lower redshift. We find that blowout is far mo re successful than early reheating, and that it may even be sufficient to s olve the angular momentum problem. We see indications that the remaining an gular momentum deficit (a factor of 5 in the blowout models) is due to curr ent limitations in the numerical method (SPH) used in our and other authors ' work. Our most successful models distinguish themselves by the fact that a large fraction of the gas is accreted very gradually, in a cooling flow f rom a surrounding hot phase rather than by mergers of massive cold clumps.