Towards a Resolution of the Galactic Spin Crisis: Mergers, Feedback, and Spin Segregation

We model in simple terms the angular-momentum problems of galaxy formation in CDM cosmologies, and identify the key elements of a scenario that may solve them. The buildup of angular momentum is modeled via dynamical friction and tidal stripping in a sequence of mergers. We demonstrate how over-cooling in incoming halos leads to a transfer of angular momentum from the baryons to the dark matter, in conflict with observations. By incorporating a simple recipe of supernova feedback, we are able to solve the problems of angular momentum in disk formation. Gas removal from the numerous small incoming halos, which merge to become the low specific angular momentum (j) component of the product, eliminates the low-j baryons. Heating and puffing-up of the gas in larger incoming halos, combined with efficient tidal stripping, reduces the angular momentum loss of baryons due to dynamical friction. Dependence of the feedback effects on the progenitor halo mass implies that the spin of baryons is typically higher for lower mass halos. The observed low baryonic fraction in dwarf galaxies is used to calibrate the characteristic velocity associated with supernova feedback, yielding ~100 km/s, within the range of theoretical expectations. We then find that the model naturally produces the observed distribution of the spin parameter among dwarf and bright disk galaxies, as well as the j profile inside these galaxies. This suggests that the model indeed captures the main features of a full scenario for resolving the spin crisis.