The Origin and Distribution of Angular Momentum in Galaxies

Cold Dark Matter with a large cosmological constant (ΛCDM) appears to fit large scale structure observations well. Of the possible small scale problems, the Central Cusps and Too Many Satellites problems now appear to be at least partly solved, so Angular Momentum has become the most serious remaining CDM problem. There are actually at least two different angular momentum problems: A. Too much transfer of angular momentum to the dark halo to make big disks, and B. Wrong distribution of spedific angular momentum to make spiral galaxies, if the baryonic material has the same angular momentum distribution as the dark matter. The angular momentum of dark matter halos, and presumably that of the galaxies they host, appears to arise largely from the orbital angular momentum of the satellites that they accrete. Since the dark and baryonic matter behave very differently in such accretion events, it is possible that the resulting angular momentum distribution of the baryons is different from that of the dark matter, as required to make the sort of galactic disks that are observed. The latest hydrodynamical simulations give some grounds for hope on this score, but much higher resolution simulations are needed. 1. Too Much Transfer of Angular Momentum to the Dark Matter to Make Big Disks