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Ja n 20 02 Resolving the Spin Crisis : Mergers and Feedback

We model in simple terms the angular momentum (J) problem of galaxy formation in CDM, and identify the key elements of a scenario that can solve it. The buildup of J is modeled via dynamical friction and tidal stripping in mergers. This reveals how over-cooling in incoming halos leads to transfer of J from baryons to dark matter (DM), in conflict with observations. By incorporating a simple recipe of supernova feedback, we match the observed J distribution in disks. Gas removal from small incoming halos, which make the low-J component of the product, eliminates the low-J baryons. Partial heating and puffing-up of the gas in larger incoming halos, combined with tidal stripping, reduces the J loss of baryons. This implies a higher baryonic spin for lower mass halos. The observed low baryonic fraction in dwarf galaxies is used to calibrate the characteristic velocity associated with supernova feedback, yielding Vfb ∼ 100 kms , within the range of theoretical expectations. The model then reproduces the observed distribution of spin parameter among dwarf and bright galaxies, as well as the J distribution inside these galaxies. This suggests that the model captures the main features of a full scenario for resolving the spin crisis.