3.4. Angular Momentum

There are two angular momentum problems [63]: (1) overcooling, and (2) the wrong distribution of angular momentum in halos. (1) Overcooling: For many years, realistic spiral galaxies did not form in hydrodynamic simulations [64]. However, it is plausible that unrealistically effective cooling ("overcooling") in the simulations was responsible for the loss of angular momentum [65]. More realistic disk galaxies have formed in recent, higher-resolution simulations including feedback [66]; an appropriate equation of state for the gas in galactic disks may play a particularly important role [67]. (2) Wrong angular momentum distribution: The standard tidal torque picture of how dark matter halos and the galaxies that they host get their angular momentum [68] suggests that the dark matter and baryons will have similar angular momentum distributions. The distribution of the specific angular momentum among the dark matter particles can be described by a simple fitting function, but disk galaxies like those seen would not form if the baryons have this same angular momentum distribution [69]. My colleagues and I have developed an alternative picture focussing on the angular momentum growth of the largest progenitor of a given halo, in which the halo's angular momentum comes mainly from the orbital angular momentum of the accreted halos, and we showed that this model accurately reproduces simulation results [70]. In this model, large angular momentum changes occur following major mergers - but the gas (which shocks) and the dark matter (which does not) would be expected to behave differently in such mergers. Steadily improving hydrodynamic simulations are being done to study these processes in the standard CDM cosmology [71], and new techniques are being developed to compare the outputs to observations [72], but it remains to be seen whether the results will be a good match to the mostly irregular galaxies observed at high redshift turning into the observed Hubble sequence of galaxies at low redshift. Our model of angular momentum growth of dark matter halos implies that the halos that have not had a recent merger have lower spin parameter (dimensionless angular momentum) than average. A perhaps surprising consequence is that the halos that host elliptical galaxies that formed from major mergers are expected to have higher angular momentum than those that host spiral galaxies (since major mergers destroy galactic disks). This is contrary to naive expectations [70, 73].