Formation of disk galaxies: feedback and the angular momentum problem

When only cooling processes are included, smoothed-particle hydrodynamical simulations of galaxy formation in a Cold Dark Matter hierarchical clustering scenario consistently produce collapsed objects that are deficient in angular momentum by a factor of about 25 relative to the disks of observed spiral galaxies. It is widely hoped that proper allowance for star formation feedback effects will resolve this discrepancy. We explore and compare the effects of including two different types of feedback event: uniform reheating of the entire Universe to 5e5 K at a redshift z~6, and gas blow-out from pregalactic gas clouds comparable to present-day dwarf galaxies at a somewhat lower redshift. We find that blow-out is far more successful than early reheating, and that it may even be sufficient to solve the angular momentum problem. We see indications that the remaining angular momentum deficit (a factor of 5 in the blow-out models) is due to current 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 from a surrounding hot phase, rather than by mergers of massive cold clumps.